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As filed with the Securities and Exchange Commission on January 15, 2016.

Registration No. 333-208781

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C.  20549

Amendment No. 1

to

FORM S-4

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

FIRST MIDWEST BANCORP, INC.

(Exact Name of Registrant as Specified in Its Charter)

One Pierce Place, Suite 1500

Itasca, Illinois 60143-9768

(630) 875-7463

(Address, Including Zip Code, and Telephone Number, Including Area Code, of Registrant’s Principal Executive Offices)

Nicholas J. Chulos

Executive Vice President, Corporate Secretary and General Counsel

First Midwest Bancorp, Inc.

One Pierce Place, Suite 1500

Itasca, Illinois 60143-9768

(630) 875-7345

(Name, Address, Including Zip Code, and Telephone Number, Including Area Code, of Agent for Service)

Copies to:

Approximate date of commencement of proposed sale to the public:  As soon as practicable after this Registration Statement becomes effective and upon completion of the merger described in the enclosed document.

If the securities being registered on this form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box: o

If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act of 1933, as amended (the “Securities Act”), check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering: o

If this form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering: o

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer or a smaller reporting company.  See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.  (Check one):

If applicable, place an X in the box to designate the appropriate rule provision relied upon in conducting this transaction:

CALCULATION OF REGISTRATION FEE

(1) Represents the estimated maximum number of shares of common stock of the registrant to be issued upon completion of the merger described in the proxy statement/prospectus contained herein.  This number is based upon the product of (x) 1,124,313, which represents the sum of 1,057,815 shares of common stock, par value $1.25 per share, of NI Bancshares Corporation outstanding as of December 23, 2015, plus 66,498 shares reserved for outstanding awards under various plans as of December 23, 2015 and issuable upon the exercise of options, multiplied by (y) 2.8858, which is the number of shares of the registrant’s common stock to be issued per share of NI Bancshares Corporation common stock under the Agreement and Plan of Merger, dated as of November 12, 2015, between NI Bancshares Corporation and First Midwest Bancorp, Inc., which is attached to the proxy statement/prospectus as Appendix A.  Under certain circumstances as described herein, First Midwest Bancorp, Inc. could issue a larger number of shares the exact number of which is not determinable at this time.  Such additional indeterminable number of shares are also hereby registered.

(2) Pursuant to Rule 457(f)(2) and (3), the proposed maximum offering price was computed on the basis of $47,131,202, which is the result of (i) $61,938,404, which is the product of the maximum number of shares of common stock that may be received by the registrant pursuant to the merger (1,124,313 shares) multiplied by the book value per share of common stock of NI Bancshares Corporation as of September 30, 2015 ($55.09), less (ii) $14,807,202, the estimated cash to be paid by the registrant in connection with the transaction.

(3)  Calculated by multiplying the estimated aggregate amount of securities to be registered by First Midwest Bancorp, Inc. by 0.0001007.

(4)   Amount previously paid in connection with First Midwest Bancorp, Inc.’s filing of Registration Statement on Form S-4 (No. 333-208781), which was filed with the Securities and Exchange Commission on December 29, 2015.

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment that specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until this Registration Statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

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Information contained herein is subject to completion or amendment. A registration statement relating to First Midwest Bancorp, Inc.’s common stock to be offered in this transaction has been filed with the Securities and Exchange Commission. These securities may not be sold, nor may offers to buy be accepted, prior to the time the registration statement becomes effective. This document shall not constitute an offer to sell, or the solicitation of an offer to buy, in any jurisdiction in which such offer, solicitation or sale would be unlawful.

PRELIMINARY—SUBJECT TO COMPLETION—DATED JANUARY 15, 2016

January 15, 2016

Dear NI Bancshares Stockholder:

You are cordially invited to attend a special meeting of the stockholders of NI Bancshares Corporation. (“NI Bancshares”), which will be held at DeKalb County Farm Bureau, located at 1350 W. Prairie Drive, Sycamore, Illinois, 60178 on February 25, 2016, at 10:30 a.m Central Time.  At the meeting, you will be asked to approve and adopt the Agreement and Plan of Merger, dated as of November 12, 2015, that NI Bancshares has entered into with First Midwest Bancorp, Inc. (“First Midwest”) and the transactions contemplated thereby, including the merger of NI Bancshares with and into First Midwest.  The accompanying proxy statement/prospectus provides you with detailed information about the merger.  In addition to being a proxy statement of NI Bancshares, this document is also the prospectus of First Midwest for shares of First Midwest common stock that will be issued to you in connection with the merger.

After careful consideration, NI Bancshares’ board of directors unanimously recommends that you vote FOR the proposal to approve and adopt the merger agreement and the transactions contemplated thereby, including the merger.

If the merger is completed, you will receive 2.8858 shares of First Midwest common stock (the “exchange ratio”), plus $13.17 in cash (without interest thereon), in exchange for each share of NI Bancshares common stock that you hold immediately prior to the completion of the merger.  NI Bancshares will have a right to terminate the merger agreement if the volume weighted average price of First Midwest common stock during a specified period before the effective time of the merger both (i) is less than $14.6055 per share and (ii) underperforms a specified index of financial institution stocks during such period by more than 15%.  However, if NI Bancshares elects to terminate the merger agreement under these circumstances, First Midwest may, but is not obligated to, elect to increase the exchange ratio or the amount of cash to be received per share of NI Bancshares common stock as provided in the merger agreement.  If this election is made, NI Bancshares may not terminate the merger agreement under these circumstances.  See “Summary—You Will Receive Cash and Shares of First Midwest Common Stock in the Merger” and “The Merger Agreement—Merger Consideration” in the accompanying proxy statement/prospectus for more information regarding the terms and conditions of the consideration to be received in the merger. 

First Midwest common stock trades on the NASDAQ Stock Market under the symbol “FMBI.”  The following table shows the implied value of the merger consideration that would be received by NI Bancshares stockholders in exchange for each share of NI Bancshares common stock if the per share price of First Midwest common stock was $18.2569, which was the per share volume weighted average price of First Midwest common stock on the NASDAQ Stock Market for the 15 trading days ending on and including November 11, 2015, the last trading day before the announcement of the merger, and if such price was $17.7848, which was the per share volume weighted average price of First Midwest common stock on the NASDAQ Stock Market for the 15 trading days ending on and including January 14, 2016, the latest practicable trading day before the printing of the accompanying proxy statement/prospectus.

(1)                 Computed as the sum of (i) the 15 day volume weighted average price of First Midwest common stock on the NASDAQ Stock Market ending on the date specified multiplied by the exchange ratio, plus (ii) the cash consideration per share of NI Bancshares common stock.

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Upon completion of the merger, each outstanding stock option (vested or unvested) to purchase NI Bancshares common stock will be cancelled and terminated in exchange for the right to receive cash, without any interest and subject to any required withholding tax, in an amount equal to the merger consideration value per share, as defined in the merger agreement, as of the trading day immediately prior to the effective date of the merger, minus the applicable exercise price per share of such outstanding stock option, multiplied by the number of shares of NI Bancshares common stock covered by such option.  Any out-of-the money stock options granted by NI Bancshares will be cancelled and no consideration will be provided.  An out-of-the money stock option is an outstanding stock option for which the exercise price per share is greater than or equal to the merger consideration value per share described above.

To the extent a director, officer or other employee holds unvested restricted stock awards immediately prior to the merger, upon completion of the merger, such awards will become fully vested and the holder shall be entitled to receive the same merger consideration for the shares of NI Bancshares common stock subject to such award as all other holders of NI Bancshares common stock.

To complete the merger, holders of a majority of the outstanding shares of NI Bancshares common stock must approve and adopt the merger agreement and the transactions contemplated thereby, including the merger.  Your vote is very important.  Whether or not you expect to attend the special meeting, please vote as soon as possible to ensure that your shares are represented at the meeting.  You may vote your shares by marking your votes on the proxy card, signing and dating it and mailing it with the envelope provided.  If you sign and return your proxy card without specifying your choice, it will be understood that you wish to have your shares voted in favor of the merger agreement and the transactions contemplated thereby, including the merger.

We encourage you to read the entire accompanying proxy statement/prospectus carefully (including any documents incorporated therein by reference).  Please pay particular attention to “Risk Factors” beginning on page 15 for a discussion of the risks related to the merger and owning First Midwest common stock after the merger.

We look forward to seeing you on February 25, 2016 in Sycamore, Illinois.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved the securities to be issued in the merger or determined if this document is accurate or adequate.  It is illegal to tell you otherwise.  The securities to be issued in the merger are not savings or deposit accounts and are not insured by the Federal Deposit Insurance Corporation or any other governmental agency.

The date of the accompanying proxy statement/prospectus is January 15, 2016, and it is first being mailed or otherwise delivered to the stockholders of NI Bancshares on or about January 22, 2016.

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NOTICE OF SPECIAL MEETING OF STOCKHOLDERS

TO BE HELD ON FEBRUARY 25, 2016

To the Stockholders of

NI Bancshares Corporation:

NOTICE IS HEREBY GIVEN that a special meeting of stockholders of NI Bancshares Corporation, a Delaware corporation (“NI Bancshares”), will be held at DeKalb County Farm Bureau, located at 1350 W. Prairie Drive, Sycamore, Illinois, 60178 on February 25, 2016 at 10:30 a.m. Central Time, for the purpose of considering and voting upon the following matters:

·                  Approval and adoption of the Agreement and Plan of Merger, dated November 12, 2015, between NI Bancshares and First Midwest Bancorp, Inc. (“First Midwest”) and the transactions contemplated thereby, including the merger of NI Bancshares with and into First Midwest, as more fully described in the accompanying proxy statement/prospectus (which we refer to as the “First Midwest merger proposal”);

·                  Approval of one or more adjournments of the special meeting, if necessary or appropriate, including adjournments to permit the further solicitation of proxies in favor of the First Midwest merger proposal (which we refer to as the “adjournment proposal”); and

·                  Transaction of such other business as may properly come before the special meeting and any adjournments or postponements thereof.

We have fixed the close of business on January 11, 2016, as the record date for determining those stockholders entitled to notice of and to vote at the special meeting and any adjournments of the special meeting.  Only NI Bancshares stockholders of record at the close of business on that date are entitled to notice of and to vote at the special meeting and any adjournments of the special meeting.  Approval of the First Midwest merger proposal requires the affirmative vote of a majority of the outstanding shares of NI Bancshares common stock.  As a result, abstentions and broker non-votes will have the same effect as votes against approval of the First Midwest merger proposal.  Approval of the adjournment proposal requires the affirmative vote of a majority of votes cast at the special meeting.

If you wish to attend the special meeting and your shares are held in the name of a broker, trust, bank or other nominee, you must bring with you a proxy or letter from the broker, trustee, bank or nominee to confirm your beneficial ownership of the shares.

Under Delaware law, NI Bancshares stockholders who do not vote in favor of the First Midwest merger proposal will have the right to seek appraisal of the fair value of their shares of NI Bancshares common stock as determined by the Delaware Court of Chancery if the merger of NI Bancshares with and into First Midwest is completed, but only if they submit a written demand for such an appraisal prior to the vote on the adoption of the First Midwest merger proposal and comply with the other Delaware law procedures explained in the accompanying proxy statement/prospectus. NI Bancshares stockholders who do not vote in favor of the First Midwest merger proposal and who submit a written demand for such an appraisal prior to the vote on the adoption of the First Midwest merger proposal and comply with the other Delaware law procedures will not receive the merger consideration.

Whether or not you plan to attend the special meeting in person, please complete, date, sign and return the enclosed proxy card in the enclosed envelope to ensure that your shares of NI Bancshares common

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stock will be represented at the special meeting if you are unable to attend.  The enclosed envelope requires no postage if mailed in the United States.  If you attend the special meeting and vote in person, your vote by ballot will revoke any proxy previously submitted.

NI Bancshares’ board of directors unanimously recommends that you vote “FOR” approval and adoption of the merger agreement and the transactions contemplated thereby, including the merger, and “FOR” the adjournment proposal.

Sycamore, Illinois

January 15, 2016

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PROXY STATEMENT OF NI BANCSHARES CORPORATION

PROSPECTUS OF FIRST MIDWEST BANCORP, INC.

Merger Proposal — Your Vote Is Important

ABOUT THIS PROXY STATEMENT/PROSPECTUS

This proxy statement/prospectus, which forms part of a registration statement on Form S-4 filed by First Midwest Bancorp, Inc. (“First Midwest”) with the Securities and Exchange Commission (the “SEC”), constitutes a prospectus of First Midwest under Section 5 of the Securities Act of 1933, as amended (the “Securities Act”), with respect to the shares of First Midwest common stock to be issued to stockholders of NI Bancshares Corporation (“NI Bancshares”) as consideration in the merger of NI Bancshares with and into First Midwest, as more fully described herein.  This proxy statement/prospectus also constitutes a proxy statement for NI Bancshares.  In addition, it constitutes a notice of meeting with respect to the special meeting of NI Bancshares stockholders.

You should rely only on the information contained in, or incorporated by reference into, this proxy statement/prospectus.  No one has been authorized to provide you with information that is different from that contained in, or incorporated by reference into, this proxy statement/prospectus.  This proxy statement/prospectus is dated January 15, 2016, and you should assume that the information in this proxy statement/prospectus is accurate only as of such date.  You should assume that the information incorporated by reference into this proxy statement/prospectus is accurate as of the date of such incorporated document.  Neither the mailing of this proxy statement/prospectus to NI Bancshares stockholders nor the issuance by First Midwest of shares of First Midwest common stock in connection with the merger will create any implication to the contrary.

This document does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction to or from any person to whom it is unlawful to make any such offer or solicitation in such jurisdiction.

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ADDITIONAL INFORMATION

This proxy statement/prospectus incorporates by reference important business and financial information about First Midwest from documents filed with the SEC that are not included in or delivered with this proxy statement/prospectus.  You can obtain any of the documents filed with or furnished to the SEC by First Midwest at no cost from the SEC’s website maintained at http://www.sec.gov.  You may also request copies of these documents, including documents incorporated by reference into this proxy statement/prospectus, at no cost by contacting First Midwest in writing at the address or by telephone as specified below:

First Midwest Bancorp, Inc.

Attention: Corporate Secretary

One Pierce Place, Suite 1500

Itasca, IL 60143

(630) 875-7463

You will not be charged for any of these documents that you request.  In order for you to receive timely delivery of the documents in advance of the special meeting of NI Bancshares stockholders, you must request the information by February 22, 2016.

See “Where You Can Find More Information.”

CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This proxy statement/prospectus, as well as First Midwest’s other filings with the SEC and NI Bancshares’ other communications with its stockholders, may contain certain “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995 (the “PSLRA”).  These statements involve known and unknown risks, uncertainties, and other factors that may cause actual results to be materially different from any results, levels of activity, performance, or achievements expressed or implied by any forward-looking statement.  These factors include, among other things, the factors listed below.

In some cases, forward-looking statements can be identified by the use of words such as “may,” “might,” “will,” “would,” “should,” “could,” “expect,” “plan,” “intend,” “anticipate,” “believe,” “estimate,” “predict,” “probable,” “potential,” “possible,” “target,” “continue,” “look forward,” or “assume” and words of similar import.  Forward-looking statements are not historical facts, but instead express only management’s beliefs, views and assumptions regarding future events, future business conditions, outcomes, and our outlook for First Midwest and NI Bancshares based on currently available information.  It is possible that actual results and events may differ, possibly materially, from the anticipated results or events indicated in these forward-looking statements.  Forward-looking statements are not guarantees of future performance, and we caution you not to place undue reliance on these statements.  Forward-looking statements are made only as of the date of this proxy statement/prospectus, and we undertake no obligation to update any forward-looking statements contained in this proxy statement/prospectus to reflect new information or events or conditions after the date hereof.

In connection with the safe harbor provisions of the PSLRA, we are hereby identifying important factors that could affect our financial performance and could cause our actual results for future periods to differ materially from any opinions or statements expressed with respect to future periods in any forward-looking statements.

Among the factors that could have an impact on our ability to achieve operating results, growth plan goals, and the beliefs expressed or implied in forward-looking statements are:

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·                  the risk that the businesses of First Midwest and NI Bancshares will not be integrated successfully or such integration may be more difficult, time consuming or costly than expected;

·                  expected revenue synergies and cost savings from the merger may not be fully realized or realized within the expected time frame;

·                  revenues following the merger may be lower than expected;

·                  deposit attrition, operating costs, customer loss and business disruption following the merger, including, without limitation, difficulties in maintaining relationships with employees, may be greater than expected;

·                  the ability to obtain governmental approvals of the merger on the proposed terms and schedule;

·                  the failure of NI Bancshares’ stockholders to approve the merger;

·                  local, regional, national and international economic conditions and the impact they may have on First Midwest and NI Bancshares and their customers and First Midwest’s and NI Bancshares’ assessment of that impact;

·                  changes in the level of non-performing assets and charge offs;

·                  changes in estimates of future reserve requirements based upon the periodic review thereof under relevant regulatory and accounting requirements;

·                  inflation, interest rate, securities market and monetary fluctuations;

·                  changes in the competitive environment among financial holding companies and banks; and

·                  changes in laws and regulations (including laws and regulations concerning taxes, banking, securities and insurance) with which First Midwest and NI Bancshares must comply.

The foregoing list of important factors may not be all inclusive, and we specifically decline to undertake any obligation to update any forward-looking statement to reflect events or circumstances after the date on which such statement is made, or to reflect the occurrence of unanticipated events.  For a further discussion of these and other risks, uncertainties and other factors applicable to First Midwest and NI Bancshares, see the section entitled “Risk Factors” in this proxy statement/prospectus and First Midwest’s other filings with the SEC incorporated by reference into this proxy statement/prospectus.

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SUMMARY

This summary highlights selected information from this proxy statement/prospectus and may not contain all the information that is important to you.  We urge you to read carefully this entire document, and the documents referenced herein, for a more complete understanding of the merger between First Midwest and NI Bancshares.  In addition, we incorporate by reference into this document important business and financial information about First Midwest.  You may obtain the information incorporated by reference in this document without charge by following the instructions in the section entitled “Where You Can Find More Information.” Each item in this summary includes a page reference directing you to a more complete description of that item.

Unless the context otherwise requires, references in this proxy statement/prospectus to “First Midwest” refer to First Midwest Bancorp, Inc., a Delaware corporation; references to “First Midwest Bank” refer to First Midwest Bank, an Illinois-state chartered bank and wholly owned subsidiary of First Midwest; references to “NI Bancshares” refer to NI Bancshares Corporation, a Delaware corporation; references to “NB&T” refer to The National Bank & Trust Company of Sycamore, a national banking association and wholly owned subsidiary of NI Bancshares; references to the “merger agreement” refer to the Agreement and Plan of Merger, dated as of November 12, 2015, between First Midwest and NI Bancshares; and references to “we,” “our” or “us” refer to First Midwest and NI Bancshares.

We Propose a Merger of NI Bancshares and First Midwest (Page 24)

We propose that NI Bancshares merge with and into First Midwest, with First Midwest as the surviving corporation (the “merger”).  As a result of the merger, the separate existence of NI Bancshares will terminate.  Following this merger, NI Bancshares’ wholly owned bank subsidiary, NB&T, will merge with and into First Midwest’s wholly owned bank subsidiary, First Midwest Bank (the “bank merger”).  We expect to complete the merger and the bank merger in the first quarter of 2016, although delays may occur.

Special Meeting of NI Bancshares (Page 21)

NI Bancshares plans to hold its special meeting of stockholders on February 25, 2016, at 10:30 a.m., Central Time at DeKalb County Farm Bureau located at 1350 W. Prairie Drive, Sycamore, Illinois, 60178.  At the special meeting you will be asked to approve and adopt the merger agreement and the transactions contemplated thereby, including the merger of NI Bancshares with and into First Midwest.

You can vote at the NI Bancshares special meeting of stockholders if you owned NI Bancshares common stock at the close of business on the record date, January 11, 2016.  As of that date, there were 1,057,815 shares of NI Bancshares common stock outstanding and entitled to vote.  You can cast one vote for each share of NI Bancshares common stock that you owned on that date.

NI Bancshares’ Board Unanimously Recommends That You Vote “FOR” the Merger (Page 22)

NI Bancshares’ board of directors (i) believes that the merger agreement and the transactions contemplated thereby, including the merger, are in the best interests of NI Bancshares and its stockholders and that the merger consideration is fair to NI Bancshares stockholders, (ii) has unanimously approved and adopted the merger agreement and (iii) unanimously recommends that NI Bancshares stockholders vote “FOR” approval and adoption of the merger agreement and the transactions contemplated thereby, including the merger.

You Will Receive Cash and Shares of First Midwest Common Stock in the Merger (Page 44)

If the merger is completed, you will receive for each share of NI Bancshares common stock you hold immediately prior to the completion of the merger 2.8858 shares of fully paid and non-assessable First Midwest common stock (the “exchange ratio”), plus $13.17 in cash (without interest thereon).  Instead of fractional shares of

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First Midwest common stock, NI Bancshares stockholders will receive a check for any fractional shares based on the price of First Midwest common stock on the trading day immediately preceding the day on which the merger occurs.

The merger consideration may be subject to adjustment if certain termination provisions, based on the market price of First Midwest common stock, are triggered and NI Bancshares’ board of directors elects to terminate the merger agreement. NI Bancshares will have a right to terminate the merger agreement if the volume weighted average price of First Midwest common stock during a specified period before the effective time of the merger both (i) is less than $14.6055 per share and (ii) underperforms a specified index of financial institution stocks during such period by more than 15%. However, if NI Bancshares elects to terminate the merger agreement under these circumstances, First Midwest may, but is not obligated to, elect to increase the exchange ratio or the amount of cash to be received per share of NI Bancshares common stock as provided in the merger agreement.  If this election is made, NI Bancshares may not terminate the merger agreement under these circumstances.  The exchange ratio is also subject to adjustment if First Midwest changes the number or kind of shares of its common stock outstanding by way of stock split, stock dividend, recapitalization, reclassification, reorganization or similar transaction.

Additionally, if certain environmental conditions and/or title defects exist with respect to NI Bancshares’ real property and the total cost to remediate and/or cure such conditions or defects is greater than $150,000, the cash received by the holders of NI Bancshares common stock in connection with the merger will be reduced by such cost up to an aggregate amount of $1,000,000.  If such cost exceeds $1,000,000, First Midwest may terminate the merger agreement or reduce the cash component of the consideration by $1,000,000 and, subject to the satisfaction of all other conditions, proceed to the closing of the merger.

The following table shows the implied value of the merger consideration that would be received by NI Bancshares stockholders in exchange for each share of NI Bancshares common stock if the per share price of a share of First Midwest common stock was $18.2569, which was the per share volume weighted average price of First Midwest common stock on the NASDAQ Stock Market for the 15 trading days ending on and including November 11, 2015, the last trading day before the announcement of the merger, and if such price was $17.7848, which was the per share volume weighted average price of First Midwest common stock on the NASDAQ Stock Market for the 15 trading days ending on and including January 14, 2016, the latest practicable trading day before the printing of this proxy statement/prospectus.

(1)                 Computed as the sum of (i) the 15 day volume weighted average price of First Midwest common stock on the NASDAQ Stock Market ending on the date specified multiplied by the exchange ratio, plus (ii) the cash consideration per NI Bancshares common share.

Upon completion of the merger, each outstanding stock option (vested or unvested) to purchase NI Bancshares common stock will be cancelled and terminated in exchange for the right to receive cash, without any interest and subject to any required withholding tax, in an amount equal to the merger consideration value per share, as defined in the merger agreement, minus the applicable exercise price per share of such outstanding stock option, multiplied by the number of shares of NI Bancshares common stock covered by such option.  Any out-of-the money stock options granted by NI Bancshares will be cancelled and no consideration will be provided.  An out-of-the money stock option is an outstanding stock option for which the exercise price per share is greater than or equal to the merger consideration value per share described above.

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To the extent a director, officer or other employee holds unvested restricted stock awards immediately prior to the merger, upon completion of the merger, such awards will become fully vested and the holder shall be entitled to receive the same merger consideration for the shares of NI Bancshares common stock subject to such award as all other holders of NI Bancshares common stock.

Tax Consequences of the Merger (Page 38)

Subject to certain circumstances described below, in the opinion of Barack Ferrazzano Kirschbaum & Nagelberg LLP (“Barack Ferrazzano”) and Chapman and Cutler LLP (“Chapman and Cutler”), for United States federal income tax purposes, the merger will be treated as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code of 1986, as amended (the “Code”).

Provided that the merger qualifies as a reorganization for United States federal income tax purposes, you may recognize gain, but you will not recognize loss, upon the exchange of your shares of NI Bancshares common stock for shares of First Midwest common stock and cash.  If the sum of the fair market value of the First Midwest common stock and the amount of cash you receive in exchange for your shares of NI Bancshares common stock exceeds the adjusted basis of your shares of NI Bancshares common stock, you will recognize taxable gain equal to the lesser of the amount of such excess or the amount of cash you receive in the exchange.  Generally, any gain recognized upon the exchange will be capital gain, and any such capital gain will be long-term capital gain if you have established a holding period of more than one year for your shares of NI Bancshares common stock.  Depending on certain facts specific to you, any gain could instead be characterized as ordinary dividend income.

For a complete description of the material United States federal income tax consequences of the transaction, including the consequences in the event of such an election, see “The Merger—Material Federal Income Tax Consequences of the Merger” on page 38.  You should consult your own tax advisor for a full understanding of the tax consequences of the merger to you.

Prohibition on NI Bancshares Dividends; First Midwest’s Dividend Policy Will Continue After the Merger (Page 58)

Except for the declaration and payment of its ordinary semi-annual dividend of $0.25 per share in accordance with the terms of the merger agreement, NI Bancshares is generally prohibited from paying cash dividends to holders of its common stock prior to completion of the merger.

First Midwest expects to continue its common stock dividend practice after the merger, but this practice is subject to the determination and discretion of First Midwest’s board of directors and may change at any time.  In the fourth quarter of 2015, First Midwest declared a quarterly cash dividend of $0.09 per share of First Midwest common stock.  In the fourth quarter of 2014 and the second quarter of 2015, payable in the first quarter of 2015 and third quarter of 2015, respectively, NI Bancshares declared a semi-annual cash dividend of $0.25 per share of NI Bancshares common stock, which equals $0.50 paid annually. For comparison, NI Bancshares stockholders would therefore receive a quarterly cash dividend following the merger equivalent to approximately $0.26 per share of NI Bancshares common stock, which equals approximately $1.04 annually, based on First Midwest’s current quarterly dividend rate of $0.09 per share and assuming that there is no adjustment to the exchange ratio as provided in the merger agreement.

The payment of dividends by First Midwest or NI Bancshares on their common stock in the future, either before or after the merger is completed, is subject to the determination and discretion of our respective boards of directors and depends on a variety of factors, including cash requirements, our financial condition and earnings, legal and regulatory considerations and other factors.

The Merger Will Be Accounted for as a Purchase (Page 41)

The merger will be treated as a purchase by First Midwest of NI Bancshares under generally accepted accounting principles (“GAAP”).

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First Midwest’s Reasons for the Merger (Page 28)

For a discussion of the factors considered by First Midwest’s board of directors in reaching its decision to approve the merger agreement and the transactions contemplated thereby, including the merger, see “The Merger—First Midwest’s Reasons for the Merger” on page 28.

NI Bancshares’ Reasons for the Merger (Page 29)

For a discussion of the factors considered by NI Bancshares’ board of directors in reaching its decision to approve the merger agreement and the transactions contemplated thereby, including the merger, see “The Merger—NI Bancshares’ Reasons for the Merger and Recommendations of the Board of NI Bancshares” on page 29.

Sheshunoff & Co. Investment Banking, L.P. Provided an Opinion to NI Bancshares’ Board Stating that, as of November 9, 2015 and Based Upon and Subject to the Factors and Assumptions Set Forth in the Opinion, the Terms of the Merger Agreement Were Fair From a Financial Point of View to NI Bancshares and NI Bancshares’ Stockholders (Page 30)

NI Bancshares engaged Sheshunoff & Co. Investment Banking, L.P. (“Sheshunoff”), to provide financial advisory services in connection with the potential sale of NI Bancshares.  Sheshunoff is an investment banking and consulting firm specializing in community bank mergers and acquisitions.  NI Bancshares selected Sheshunoff as its financial advisor on the basis of its experience and expertise in representing community banks in similar transactions and their familiarity with NI Bancshares.

On November 9, 2015, the date NI Bancshares’ board of directors approved the merger, Sheshunoff provided its oral and written opinion to NI Bancshares’ board of directors that, as of that date and subject to a number of factors and assumptions, the terms of the merger agreement were fair from a financial point of view to NI Bancshares and NI Bancshares’ stockholders.  The full text of Sheshunoff’s written opinion is attached to this proxy statement/prospectus as Appendix B.  We encourage you to read this opinion carefully and in its entirety.  The Sheshunoff opinion is not a recommendation as to how any NI Bancshares stockholders should vote or act with respect to the merger.

NI Bancshares and Sheshunoff have entered into an agreement relating to the services to be provided by Sheshunoff in connection with the merger.  NI Bancshares paid Sheshunoff a cash fee of $15,000 upon execution of their engagement letter.  NI Bancshares paid Sheshunoff a cash fee of $50,000 upon the issuance of the Sheshunoff fairness opinion and, upon the closing of the merger, has agreed to pay Sheshunoff a transaction fee in an amount equal to 0.95% of the aggregate consideration value thereof. Both the $15,000 and $50,000 cash fees are to be credited toward the transaction fee. NI Bancshares has also agreed to reimburse Sheshunoff for its reasonable out-of-pocket expenses, and to indemnify Sheshunoff against certain liabilities.

NI Bancshares’ Directors and Executive Officers May Have Interests in the Merger that Differ from Your Interests (Page 42)

Some of NI Bancshares’ directors and executive officers have interests in the merger other than their interests as stockholders, including:

·                  Per the terms of certain NI Bancshares employment agreements and benefit plans, NI Bancshares executive officers may become entitled to change in control, severance, or other payments upon the occurrence of the merger.

·                  NI Bancshares and Michael A. Cullen, its Chief Executive Officer (the “CEO”), are parties to an employment agreement that provides for certain severance benefits in the event of a qualifying termination of employment in connection with a change in control of NI Bancshares.  In such instance, Mr. Cullen will be entitled to a payment equal to three times the sum of his current salary

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plus his average bonus for the preceding three years.  In addition, he would be entitled to company paid insurance coverage for 18 months.  Following termination for any reason, Mr. Cullen would be subject to non-competition and non-solicitation restrictions for 18 months.  Additionally, upon the occurrence of a change in control, Mr. Cullen would become vested in company contributions to his deferred compensation account.  In connection with the execution of the merger agreement, First Midwest and Mr. Cullen entered into a letter agreement outlining Mr. Cullen’s employment by First Midwest as its Regional President of DeKalb County and his compensation arrangements that will become effective immediately following the closing of the merger.  Under the letter agreement, First Midwest and Mr. Cullen have agreed to enter into a new employment agreement that will become effective immediately following the closing of the merger.  The letter agreement provides for the termination of his prior employment agreement with NI Bancshares (other than the past post-employment restrictions) and of his participation in the NI Bancshares Executive Change in Control Severance Plan (“CIC Severance Plan”), described below, following the merger. In addition, the letter agreement sets forth the general terms and conditions of his employment following the merger.  In recognition of termination of his existing agreement with NI Bancshares, continuation of the post-employment restrictions contained therein and as a retention incentive, the new employment agreement with First Midwest will provide for a cash payment to Mr. Cullen by First Midwest of $447,516 following the merger and for a restricted stock award by First Midwest with a grant date value of $447,516 and a vesting period of three years.  Such aggregate amount is approximately 80% of the severance benefit Mr. Cullen would have been entitled to receive upon a qualifying termination in connection with the merger under the terms of his existing employment agreement with NI Bancshares.

·                  Certain executive officers of NI Bancshares are participants in the CIC Severance Plan.  Under the CIC Severance Plan, if a covered officer is terminated without cause, or resigns for good reason, the officer will be entitled to severance payments ranging from 12 to 18 months of salary plus COBRA insurance continuation for 12 to 18 months with the officer responsible for the same level of payments as active employees.  Following termination of employment, the covered officer will be subject to non-competition and non-solicitation restrictions for 12 to 18 months, corresponding to the length of severance benefits.  First Midwest has agreed to honor the obligations of NI Bancshares under the CIC Severance Plan after the effective time of the merger.  In connection with the execution of the merger agreement, First Midwest entered into letter agreements with two executive officers and four other officers setting forth employment and compensation arrangements that will become effective as of the effective date of the merger.  The letter agreements provide for the officers’ waiver of participation in the CIC Severance Plan, in return for a sign-on bonus falling in the range of $25,000 to $50,000.

·                  To the extent a director or executive officer holds unvested restricted stock awards, upon completion of the merger, such awards shall become fully earned, and vested per the terms of the NI Bancshares Corporation 2000 Stock Incentive Plan and the NI Bancshares 2010 Equity Incentive Plan (the “NI Bancshares Stock Plans”) and will be converted to cash and First Midwest common stock, and to the extent a director or officer holds vested or unvested stock options, the options will be cancelled and terminated in exchange for a cash payment discussed in the “You Will Receive Cash and Shares of First Midwest Common Stock in the Merger” section of this proxy statement/prospectus above.  As of the date of the merger agreement, directors and executive officers, as a group, held unvested restricted stock awards with respect to 4,607 shares of NI Bancshares common stock and 42,166 vested stock options.

·                  Under the merger agreement, First Midwest has agreed to indemnify the directors and officers of NI Bancshares against liabilities arising out of actions or omissions occurring at or before the completion of the merger.

·                  The merger agreement also provides that, subject to certain limitations, First Midwest will maintain directors’ and officers’ liability insurance for a period of six years after the merger is completed that provides at least the same coverage and amounts, and contains terms and conditions no less advantageous, as that coverage currently provided by NI Bancshares.  See “The Merger—Interests of Certain Persons in the Merger” on page 42.

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NI Bancshares’ board of directors knew about these additional interests and considered them when they adopted the merger agreement and approved the merger.

NI Bancshares Stockholders Have Dissenters’ Rights of Appraisal (Page 58)

If you are a stockholder of NI Bancshares, you may elect to dissent from the merger and exercise appraisal rights by following the procedures set forth in Section 262 of the General Corporation Law of the State of Delaware (the “DGCL”).  For more information regarding your right to dissent from the merger and exercise appraisal rights, please see “The Merger Agreement—Dissenters’ Rights of Appraisal of NI Bancshares Stockholders” on page 58.  We have also attached a copy of the relevant provisions of Section 262 of the DGCL as Appendix C to this proxy statement/prospectus.

We Have Agreed When and How NI Bancshares Can Consider Third-Party Acquisition Proposals (Page 49)

We have agreed that NI Bancshares will not initiate or solicit proposals from other parties regarding acquiring NI Bancshares or its businesses.  In addition, we have agreed that NI Bancshares will not engage in negotiations with or provide confidential information to a third party regarding acquiring NI Bancshares or its businesses.  However, if NI Bancshares receives an acquisition proposal from a third party, NI Bancshares can participate in negotiations with and provide confidential information to the third party if, among other steps, NI Bancshares’ board of directors concludes in good faith that the proposal is superior to First Midwest’s merger proposal.  NI Bancshares’ receipt of a superior proposal or participation in such negotiations does not give NI Bancshares the right to terminate the merger agreement.

Merger Approval Requires the Affirmative Vote of the Holders of a Majority of the Outstanding Shares of NI Bancshares  (Page 22)

In order to approve the merger agreement and the transactions contemplated thereby, including the merger, the holders of a majority of the outstanding shares of NI Bancshares common stock as of the record date of January 11, 2016 must vote in favor of that matter.  As of that date, NI Bancshares’ directors and executive officers and their affiliates held approximately 21% of the outstanding shares of NI Bancshares common stock entitled to vote at the special meeting.  All of the directors and certain executive officers of NI Bancshares and NB&T, collectively holding an aggregate 220,654 shares of NI Bancshares common stock (or approximately 21% of the outstanding shares) as of the record date, have signed voting agreements with First Midwest agreeing to vote for approval of the merger agreement and the transactions contemplated thereby, including the merger.

Per the terms of the NI Bancshares Stock Plans, and award agreements thereunder, holders of stock options do not have voting rights with respect to the shares underlying unexercised stock options, whereas recipients of restricted stock awards are entitled to vote on behalf of underlying shares, even while subject to vesting requirements, from the date of grant until such awards are forfeited.  Holders of NI Bancshares restricted stock will vote in the same manner as holders of NI Bancshares common stock, as discussed above.  As of the record date, participants in the NI Bancshares Stock Plans held approximately 23% of the shares entitled to vote at the special meeting.

For a list of the number of shares of NI Bancshares common stock held by each director and executive officer of NI Bancshares and each stockholder that is known to NI Bancshares as of the date of this proxy statement/prospectus to beneficially own more than 5% percent of the outstanding shares of NI Bancshares common stock, see “Security Ownership of NI Bancshares Directors, Named Executive Officers and Certain Beneficial Owners” on page 75.

Certain Stockholders of NI Bancshares Have Agreed to Vote Their Shares “FOR” the Merger (Page 63 and Appendix A, Annex 1-B)

As an inducement to and condition of First Midwest’s willingness to enter into the merger agreement, all of the directors and certain officers of NI Bancshares and NB&T who beneficially owned in the aggregate

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approximately 21% of NI Bancshares’ outstanding common stock as of November 12, 2015 entered into voting agreements, pursuant to which, among other things, they agreed to vote all of their shares of NI Bancshares common stock in favor of approval of the merger agreement and the transactions contemplated thereby, including the merger, other matters required to be approved or adopted to effect the merger and any other transactions contemplated by the merger agreement.

We Must Meet Several Conditions to Complete the Merger (Page 53)

Our obligations to complete the merger depend on a number of conditions being met.  These include:

·                  the approval of the merger agreement and the merger by NI Bancshares stockholders;

·                  the receipt of the required approvals of federal and state regulatory authorities;

·                  the listing on the NASDAQ Stock Market of the shares of First Midwest common stock to be issued in the merger;

·                  the absence of any government action or other legal restraint or prohibition that would prohibit the merger or make it illegal;

·                  the representations and warranties of the other party to the merger agreement being true and correct in all material respects (except for representations and warranties qualified by the words “material” or “Material Adverse Effect,” and certain representations and warranties regarding NI Bancshares’ capitalization, which are required to be true in all respects), and the other party to the merger agreement having performed in all material respects all its obligations and complied in all material respects with all of its agreements and covenants under the merger agreement;

·                  the receipt of legal opinions that, for United States federal income tax purposes, the merger will be treated as a reorganization and that both NI Bancshares and First Midwest will be a party to that reorganization.  These opinions will be based on customary assumptions and on factual representations made by First Midwest and NI Bancshares and will be subject to various limitations;

·                  with regard to First Midwest’s obligation (but not NI Bancshares’), the receipt of a legal opinion from NI Bancshares’ outside counsel, Barack Ferrazzano, as to certain corporate matters, including NI Bancshares’ due incorporation and legal standing, the legal status of NI Bancshares’ capital stock and the due authorization and execution of the merger agreement;

·                  with regard to First Midwest’s obligation (but not NI Bancshares’), the number of dissenting shares must not exceed 5% of NI Bancshares common stock;

·                  with regard to First Midwest’s obligation (but not NI Bancshares’), the receipt of all required third-party approvals;

·                  with regard to First Midwest’s obligation (but not NI Bancshares’), NI Bancshares’ closing tangible equity, as defined in the merger agreement, must be greater than or equal to $53,000,000;

·                  with regard to First Midwest’s obligation (but not NI Bancshares’), NB&T’s loan loss reserve must be greater than or equal to 0.90% of NB&T’s net extensions of credit;

·                  with regard to First Midwest’s obligation (but not NI Bancshares’), the 10-day average balance of NB&T’s consolidated deposits must be no less than $515,000,000 for the 10-day period ending on the day immediately prior to the closing date;

·                  with regard to First Midwest’s obligation (but not NI Bancshares’), NI Bancshares’ consolidated total loans (excluding loans held for sale) must be greater than or equal to $350,000,000;

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·                  with regard to First Midwest’s obligation (but not NI Bancshares’), the environmental and title review process of NI Bancshares’ real property set forth in the merger agreement shall be completed in accordance with the provisions of the merger agreement;

·                  with regard to First Midwest’s obligation (but not NI Bancshares’), the receipt by First Midwest of the resignations, effective as of the effective time of the merger, of each director and officer of NI Bancshares and each director and executive officer of NB&T unless such individual has entered into an alternative agreement with First Midwest for continued employment following the effective time of the merger;

·                  with regard to First Midwest’s obligation (but not NI Bancshares’), the receipt by First Midwest of a certificate by NI Bancshares stating that it and NB&T are not and have not been United States real property holding corporations; and

·                  with regard to First Midwest’s obligation (but not NI Bancshares’), the receipt by First Midwest of a certificate by NI Bancshares stating a material adverse effect, as defined in the merger agreement, has not occurred to NI Bancshares.

Where the law permits, either of First Midwest or NI Bancshares could choose to waive a condition to its obligation to complete the merger even when that condition has not been satisfied.  We cannot be certain when, or if, the conditions to the merger will be satisfied or waived, or that the merger will be completed.  Although the merger agreement allows us to waive the tax opinion condition, we do not currently anticipate doing so.

We Must Obtain Regulatory Approvals to Complete the Merger (Page 57)

The merger and the related transactions require approval from the Board of Governors of the Federal Reserve System (the “Federal Reserve”).  The Federal Reserve has approved the merger and the related transactions, including the bank merger.  Notice of the bank merger must also be given to the Office of the Comptroller of the Currency (the “OCC”), and such notice has been provided.  The bank merger must also be approved by the Illinois Department of Financial and Professional Regulation (the “IDFPR”), and an application has been filed.

We May Terminate the Merger Agreement (Page 55)

We can mutually agree at any time to terminate the merger agreement without completing the merger, even if NI Bancshares stockholders have approved the merger agreement and the merger.  Also, either of us can decide, without the consent of the other, to terminate the merger agreement in certain circumstances, including:

·                  if a material adverse effect, as defined in the merger agreement, occurs with respect to the other party;

·                  if there is a final denial of a required regulatory approval or an application for a required regulatory approval has been withdrawn upon the request or recommendation of the applicable governmental authority and such governmental authority would not accept the refiling of such application;

·                  if the merger is not completed on or before November 12, 2016; or

·                  if there is a continuing breach of the merger agreement by a party, and the breaching party has not cured the breach within 15 days’ written notice to the breaching party, as long as that breach would allow the non-breaching party not to complete the merger.

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In addition, First Midwest may terminate the merger agreement:

·                  if NI Bancshares stockholders fail to approve the merger agreement and the transactions contemplated thereby, including the merger;

·                  if NI Bancshares’ board of directors fails to recommend approval of the merger agreement and the transactions contemplated thereby, including the merger, to its stockholders, or withdraws or materially and adversely modifies its recommendation;

·                  if NI Bancshares’ board of directors recommends an acquisition proposal other than the merger, or if NI Bancshares’ board of directors negotiates or authorizes negotiations with a third party regarding an acquisition proposal other than the merger and those negotiations continue for at least 10 business days;

·                  if NI Bancshares has breached its covenant not to solicit or encourage inquiries or proposals with respect to any acquisition proposal, in circumstances not permitted under the merger agreement;

·                  if the aggregate cost to remediate and/or cure environmental conditions and/or title defects with respect to NI Bancshares’ real property exceeds $1,000,000; or

·                  if the number of dissenting shares exceeds 5% of the outstanding shares of NI Bancshares common stock.

Further, NI Bancshares may terminate the merger agreement if the volume weighted average price of First Midwest common stock during a specified period before the effective time of the merger both (i) is less than $14.6055 per share and (ii) underperforms a specified index of financial institution stocks during such period by more than 15%.  However, if NI Bancshares elects to terminate the merger agreement under these circumstances, First Midwest may, but is not obligated to, elect to increase the exchange ratio or the amount of cash to be received per share of NI Bancshares common stock as provided in the merger agreement.  If this election is made, NI Bancshares may not terminate the merger agreement under these circumstances.

Whether or not the merger is completed, we will each pay our own fees and expenses, except that we will each pay one-half of the costs and expenses that we incur in preparing, printing and mailing this proxy statement/prospectus and filing fees paid in connection with the registration statement of which the proxy statement/prospectus forms a part and all applications for government approvals, except fees paid to counsel, financial advisors and accountants.

The merger agreement also provides that NI Bancshares must pay First Midwest a fee and reimburse expenses in certain situations.  In particular, NI Bancshares will pay First Midwest a fee of $2,800,000 if the following occurs on or prior to the termination of the merger agreement or the 18 month anniversary of the termination of the merger agreement in certain circumstances set forth in the merger agreement:

·                  NI Bancshares’ board of directors submits the merger agreement and the transactions contemplated thereby, including the merger, to NI Bancshares stockholders without a recommendation for approval or with material and adverse conditions on such approval, or withdraws or materially and adversely modifies its recommendation;

·                  NI Bancshares enters into an agreement to engage in a competing acquisition proposal with any person other than First Midwest or any of First Midwest’s subsidiaries;

·                  NI Bancshares authorizes, recommends or proposes (or publicly announces its intention to authorize, recommend or propose) an agreement to engage in a competing acquisition proposal with any person other than First Midwest or its subsidiaries or NI Bancshares’ board of directors

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recommends that NI Bancshares stockholders approve or accept such a competing acquisition proposal;

·                  any person, other than First Midwest or its subsidiaries, acquires beneficial ownership or the right to acquire beneficial ownership of 20% or more of the outstanding shares of any class or series of NI Bancshares stock;

·                  NI Bancshares fails to convene a stockholder meeting to approve the merger agreement and the transactions contemplated thereby, including the merger, within 45 days of the effectiveness of the registration statement of which this proxy statement/prospectus is a part; or

·                  NI Bancshares breaches its covenant not to solicit or encourage inquiries or proposals with respect to any acquisition proposal in circumstances not permitted under the merger agreement, which covenant is described below under “The Merger Agreement—Acquisition Proposals by Third Parties” on page 49.

We May Amend or Waive Merger Agreement Provisions (Page 56)

At any time before completion of the merger, either First Midwest or NI Bancshares may, to the extent legally allowed, waive in writing compliance by the other with any provision contained in the merger agreement.  However, once NI Bancshares stockholders have voted on the proposed transaction, no waiver of any condition may be made that would require further approval by NI Bancshares stockholders unless that approval is obtained.

First Midwest may also change the structure of the merger by providing at least five days’ notice before NI Bancshares stockholders vote on the proposed transaction, so long as any change does not:  (1) change the amount or type of consideration to be received by NI Bancshares stockholders; (2) adversely affect the timing of or capability of completion of the merger; (3) adversely affect the tax consequences of the merger to NI Bancshares stockholders; or (4) cause or could not be reasonably expected to cause any of the conditions to complete the merger to be incapable of being satisfied.

The Rights of NI Bancshares Stockholders Following the Merger Will be Different (Page 70)

The rights of First Midwest stockholders are governed by Delaware law and by First Midwest’s restated certificate of incorporation and amended and restated by-laws.  The rights of NI Bancshares stockholders are also governed by Delaware law, and by NI Bancshares’ amended and restated certificate of incorporation and by-laws.  Upon our completion of the merger, the rights of both stockholder groups will be governed by Delaware law and First Midwest’s restated certificate of incorporation and amended and restated by-laws.

Information About First Midwest and NI Bancshares (Page 66)

First Midwest Bancorp, Inc.

One Pierce Place, Suite 1500

Itasca, Illinois 60143

(630) 875-7463

First Midwest is a Delaware corporation headquartered in the Chicago suburb of Itasca, Illinois.  It is one of Illinois’ largest independent publicly-traded bank holding companies.  First Midwest’s principal subsidiary, First Midwest Bank, and other affiliates provide a full range of business, middle market and retail banking as well as wealth management and private banking services to commercial and industrial, commercial real estate, municipal and consumer customers through over 100 locations in metropolitan Chicago, northwest Indiana, central and western Illinois, and eastern Iowa.  At September 30, 2015, First Midwest had consolidated total assets of approximately $9.9 billion.  First Midwest common stock trades on the NASDAQ Stock Market under the symbol “FMBI.”

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NI Bancshares Corporation

230 West State Street

Sycamore, Illinois  60178

(815) 895-2125

NI Bancshares, a Delaware corporation incorporated in 1990, is a registered bank holding company headquartered in Sycamore, Illinois. Its primary business is operating its bank subsidiary, NB&T, a national banking association headquartered in Sycamore, Illinois.  The predecessor to NB&T was founded in 1867 and NB&T currently serves local businesses and individuals through a full range of services including business and retail banking, trust and wealth management, farm management and financial planning.  It provides these financial services through ten banking locations throughout DeKalb, LaSalle and Kane Counties in Illinois.  At September 30, 2015, NB&T had approximately $680 million in total assets, $606 million in deposits and $415 million in loans.  NB&T also had over $700 million in trust assets under administration at September 30, 2015.

NI Bancshares common stock is not registered under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), and, accordingly, the company does not file periodic or current reports with the SEC.  NI Bancshares common stock is traded on the OTC Pink market place under the symbol “NIBA”.

See “Information about First Midwest and NI Bancshares” on page 66.

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SELECTED CONSOLIDATED FINANCIAL DATA OF FIRST MIDWEST

You should read the selected consolidated financial data set forth below in conjunction with First Midwest’s Management’s Discussion and Analysis of Financial Condition and Results of Operations and the First Midwest Consolidated Financial Statements and related notes incorporated by reference into this proxy statement/prospectus. The financial data as of and for the fiscal years ended December 31, 2014, 2013, 2012, 2011 and 2010 is derived from First Midwest’s audited financial statements. The financial data as of and for the nine month-periods ended September 30, 2015 and 2014 is derived from First Midwest’s unaudited financial statements incorporated by reference into this proxy statement/prospectus, which have been prepared on the same basis as First Midwest’s audited financial statements. See “Where You Can Find More Information.” First Midwest’s historical results may not be indicative of First Midwest’s future performance. In addition, results for the nine-month periods ended September 30, 2015 and 2014 may not be indicative of the results that may be expected for the full fiscal year or future periods.

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Footnotes to Selected Consolidated Financial Data of First Midwest

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SELECTED CONSOLIDATED FINANCIAL DATA OF NI BANCSHARES

You should read the selected consolidated financial data set forth below in conjunction with NI Bancshares’ Consolidated Financial Statements and related notes included elsewhere in this proxy statement/prospectus. The financial data as of and for the fiscal years ended December 31, 2014, 2013, 2012, 2011 and 2010 is derived from NI Bancshares’ audited financial statements.  The financial data as of and for the nine month-periods ended September 30, 2015 and 2014 is derived from NI Bancshares’ unaudited financial statements, which have been prepared on the same basis as NI Bancshares’ audited financial statements. NI Bancshares’ historical results may not be indicative of NI Bancshares’ future performance. In addition, results for the nine-month periods ended September 30, 2015 and 2014 have not been audited and may not be indicative of the results that may be expected for the full fiscal year or future periods.

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RISK FACTORS

In addition to the other information contained in or incorporated by reference into this proxy statement/prospectus, including the matters addressed under the heading “Cautionary Statement Regarding Forward-Looking Statements,” you should carefully consider the following risk factors in deciding how to vote on the proposals presented in this proxy statement/prospectus.  You should also consider the other information in, and the other documents incorporated by reference into, this proxy statement/prospectus, including in particular the risk factors associated with First Midwest’s business contained under the heading “Risk Factors” in First Midwest’s Annual Report on Form 10-K for the year ended December 31, 2014.  See “Where You Can Find More Information.”

Because the market price of First Midwest common stock will fluctuate, NI Bancshares stockholders cannot be certain of the market value of the merger consideration they will receive.

Upon completion of the merger, you will receive for each share of NI Bancshares common stock you hold immediately prior to the completion of the merger 2.8858 shares of fully paid and non-assessable First Midwest common stock, plus $13.17 in cash (without interest thereon).  Any change in the market price of First Midwest common stock prior to completion of the merger will affect the value of any shares of First Midwest common stock you receive as consideration in the merger.  The market price of First Midwest common stock may fluctuate as a result of a variety of factors, including general market and economic conditions, changes in our respective businesses, operations and prospects, and regulatory considerations.  Many of these factors are outside our control.  Accordingly, at the time of the special meeting, you will not know or be able to calculate the market price of First Midwest common stock you will receive upon completion of the merger.

NI Bancshares will be subject to business uncertainties and contractual restrictions while the merger is pending.

Uncertainty about the effect of the merger on employees and customers may have an adverse effect on NI Bancshares and consequently on First Midwest.  These uncertainties may impair NI Bancshares’ ability to attract, retain and motivate key personnel until the merger is completed, and could cause customers and others that deal with NI Bancshares to seek to change existing business relationships with NI Bancshares.  Retention of certain employees may be challenging during the pendency of the merger, as employees may experience uncertainty about their future roles with First Midwest.  If key employees depart because of issues relating to the uncertainty and difficulty of integration or a desire not to remain with First Midwest, First Midwest’s business following the merger could be harmed.  In addition, the merger agreement restricts NI Bancshares from making certain acquisitions and taking other specified actions without the consent of First Midwest, and generally requires NI Bancshares to continue its operations in the ordinary course, until the merger occurs.  These restrictions may prevent NI Bancshares from pursuing attractive business opportunities that may arise prior to the completion of the merger.  Please see the section entitled “The Merger Agreement—Conduct of Business Pending the Merger” on page 48 for a description of the restrictive covenants to which NI Bancshares is subject.

Combining our two companies may be more difficult, costly or time-consuming than we currently expect, and we may fail to realize the anticipated benefits and cost savings of the merger.

First Midwest and NI Bancshares have operated and, until the completion of the merger, will continue to operate, independently.  The success of the merger, including anticipated benefits and cost savings, will depend, in part, on First Midwest’s ability to successfully combine and integrate the NI Bancshares business into its own in a manner that permits growth opportunities and does not materially disrupt existing customer relationships or result in decreased revenues due to loss of customers.  It is possible that the integration process could result in the loss of key employees, the disruption of either company’s ongoing business or inconsistencies in standards, controls, procedures and policies that adversely affect our ability to maintain relationships with customers and employees.  As with any merger of banking institutions, there also may be business disruptions that cause us to lose customers or cause customers to take their deposits out of our banks.  The success of the combined company following the merger may

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depend in part on the ability of First Midwest to integrate the two businesses, business models and cultures.  If First Midwest experiences difficulties in the integration process, including those listed above, First Midwest may fail to realize the anticipated benefits of the merger in a timely manner or at all.  First Midwest’s business or results of operations or the value of its common stock may be materially and adversely affected as a result.

The market price of First Midwest common stock after the merger may be affected by factors different from those currently affecting First Midwest common stock.

The businesses of First Midwest and NI Bancshares differ in some respects and, accordingly, the results of operations of the combined company and the market price of First Midwest common stock after the merger may be affected by factors different from those currently affecting the independent results of operations of each of First Midwest or NI Bancshares.  For a discussion of the business of First Midwest and of certain factors to consider in connection with the business of First Midwest, see the documents incorporated by reference into this proxy statement/prospectus and referred to under “Where You Can Find More Information,” including in particular the section titled “Risk Factors” in First Midwest’s Annual Report on Form 10-K for the year ended December 31, 2014.

Regulatory approvals may not be received, may take longer than expected or may impose conditions that are not presently anticipated or that could have an adverse effect on the combined company following the merger.

Before the merger and the bank merger may be completed, First Midwest and NI Bancshares must obtain approvals from the Federal Reserve and the IDFPR.  In addition, notice of the bank merger must be provided to the OCC.  Other approvals, waivers or consents from regulators may also be required.  In determining whether to grant these approvals the regulators consider a variety of factors, including the regulatory standing of each party and the factors described under “The Merger—Regulatory Approvals Required for the Merger” on page 57. An adverse development in either party’s regulatory standing or these factors could result in an inability to obtain approval or delay their receipt.  These regulators may impose conditions on the completion of the merger or the bank merger or require changes to the terms of the merger or the bank merger.  Such conditions or changes could have the effect of delaying or preventing completion of the merger or the bank merger or imposing additional costs on or limiting the revenues of the combined company following the merger and the bank merger, any of which might have an adverse effect on the combined company following the merger.  See “The Merger Agreement—Regulatory Approvals Required for the Merger” on page 57.  Regulatory approvals could also be adversely impacted based on the status of any ongoing investigation of either party or its customers, including subpoenas to provide information or investigations, by a federal, state or local governmental agency.  We cannot guarantee that we will be able to obtain all required regulatory approvals, the timing of those approvals or whether any conditions will be imposed.

Some NI Bancshares directors and officers may have interests and arrangements that may have influenced their decisions to support or recommend that you approve the merger.

The interests of some of the directors and officers of NI Bancshares may be different from those of NI Bancshares stockholders generally, and directors and officers of NI Bancshares may be participants in arrangements that are different from, or in addition to, those of NI Bancshares stockholders, including:

·                  Per the terms of certain NI Bancshares employment agreements and benefit plans, NI Bancshares executive officers may become entitled to change in control, severance, or other payments upon the occurrence of the merger.

·                  NI Bancshares and its CEO, Mr. Cullen, are parties to an employment agreement that provides for certain severance benefits in the event of a qualifying termination of employment in connection with a change in control of NI Bancshares.  In such instance, Mr. Cullen will be entitled to a payment equal to three times the sum of his current salary plus his average bonus for the preceding three years.  In addition, he would be entitled to company paid insurance coverage for 18 months.  Following termination for any reason, Mr. Cullen would be subject to non-competition and non-solicitation restrictions for 18 months.  Additionally, upon the occurrence of a change in

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control, Mr. Cullen would become vested in company contributions to his deferred compensation account.  In connection with the execution of the merger agreement, First Midwest and Mr. Cullen entered into a letter agreement outlining Mr. Cullen’s employment by First Midwest as its Regional President of DeKalb County and his compensation arrangements that will become effective immediately following the closing of the merger.  Under the letter agreement, First Midwest and Mr. Cullen have agreed to enter into a new employment agreement that will become effective immediately following the closing of the merger. The letter agreement provides for the termination of his prior employment agreement with NI Bancshares (other than the past post-employment restrictions) and of his participation in the CIC Severance Plan, described below, following the merger. In addition, the letter agreement sets forth the general terms and conditions of his employment following the merger.  In recognition of termination of his existing agreement with NI Bancshares, continuation of the post-employment restrictions contained therein and as a retention incentive, the new employment agreement with First Midwest will provide for a cash payment to Mr. Cullen by First Midwest of $447,516 following the merger and for a restricted stock award by First Midwest with a grant date value of $447,516 and a vesting period of three years.  Such aggregate amount is approximately 80% of the severance benefit Mr. Cullen would have been entitled to receive upon a qualifying termination in connection with the merger under the terms of his existing employment agreement with NI Bancshares.

·                  Certain executive officers of NI Bancshares are participants in the CIC Severance Plan.  Under the CIC Severance Plan, if a covered officer is terminated without cause, or resigns for good reason, the officer will be entitled to severance payments ranging from 12 to 18 months of salary plus COBRA insurance continuation for 12 to 18 months with the officer responsible for the same level of payments as active employees.  Following termination of employment, the covered officer will be subject to non-competition and non-solicitation restrictions for 12 to 18 months, corresponding to the length of severance benefits.  First Midwest has agreed to honor the obligations of NI Bancshares under the CIC Severance Plan after the effective time of the merger.  In connection with the execution of the merger agreement, First Midwest entered into letter agreements with two executive officers and four other officers setting forth employment and compensation arrangements that will become effective as of the effective date of the merger.  The letter agreements provide for the officers’ waiver of participation in the CIC Severance Plan, in return for a sign-on bonus falling in the range of $25,000 to $50,000.

·                  To the extent a director or executive officer holds unvested restricted stock awards, upon completion of the merger, such awards shall become fully earned, and vested per the terms of the NI Bancshares Corporation 2000 Stock Incentive Plan and the NI Bancshares Stock Plans and will be converted to cash and First Midwest common stock, and to the extent a director or officer holds vested or unvested stock options, the options will be cancelled and terminated in exchange for a cash payment discussed in the “You Will Receive Cash and Shares of First Midwest Common Stock in the Merger” section of this proxy statement/prospectus above.  As of the date of the merger agreement, directors and executive officers, as a group, held unvested restricted stock awards with respect to 4,607 shares of NI Bancshares common stock and 42,166 vested stock options.

·                  Under the merger agreement, First Midwest has agreed to indemnify the directors and officers of NI Bancshares against liabilities arising out of actions or omissions occurring at or before the completion of the merger.

·                  The merger agreement also provides that, subject to certain limitations, First Midwest will maintain directors’ and officers’ liability insurance for a period of six years after the merger is completed that provides at least the same coverage and amounts, and contains terms and conditions no less advantageous, as that coverage currently provided by NI Bancshares.  See “The Merger—Interests of Certain Persons in the Merger” on page 42.

These interests are described in more detail in “The Merger—Interests of Certain Persons in the Merger” on page 42.

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The merger agreement limits NI Bancshares’ ability to pursue alternatives to the merger.

The merger agreement contains provisions that limit NI Bancshares’ ability to solicit, encourage or discuss competing third-party proposals to acquire all or a significant part of NI Bancshares.  These provisions, which include a $2,800,000 termination fee, might discourage a potential competing acquiror that might have an interest in acquiring all or a significant part of NI Bancshares from considering or proposing that acquisition even if it were prepared to pay consideration with a higher per share market price than that proposed in the merger, or might result in a potential competing acquiror proposing to pay a lower per share price to acquire NI Bancshares than it might otherwise have proposed to pay.

Termination of the merger agreement could negatively impact First Midwest or NI Bancshares.

In the event the merger agreement is terminated, First Midwest’s or NI Bancshares’ business may be adversely impacted by the failure to pursue other beneficial opportunities due to the focus of management on the merger.  The market price of First Midwest common stock might decline to the extent that the current market price reflects a market assumption that the merger will be completed.  If the merger agreement is terminated and NI Bancshares’ board of directors seeks another merger or business combination, NI Bancshares stockholders cannot be certain that NI Bancshares will be able to find a party willing to offer equivalent or more attractive consideration than the merger consideration provided in the merger.  If the merger agreement is terminated under certain circumstances, NI Bancshares may be required to pay First Midwest a termination fee of $2,800,000.  See “The Merger Agreement—Termination of the Merger Agreement” on page 55.

If the merger is not completed, First Midwest and NI Bancshares will have incurred substantial expenses without realizing the expected benefits of the merger.

Each of First Midwest and NI Bancshares has incurred and will incur substantial expenses in connection with the negotiation and completion of the transactions contemplated by the merger agreement, as well as the costs and expenses of filing, printing and mailing this proxy statement/prospectus and all filing and other fees paid to the SEC in connection with the merger.  If the merger is not completed, First Midwest and NI Bancshares would have to recognize these expenses without realizing the expected benefits of the merger.

Holders of NI Bancshares common stock will have a reduced ownership and voting interest after the merger and will exercise less influence over management.

NI Bancshares stockholders currently have the right to vote in the election of the board of directors of NI Bancshares and on other matters affecting NI Bancshares.  Upon the completion of the merger, each NI Bancshares stockholder who receives shares of First Midwest common stock will become a stockholder of First Midwest with a percentage ownership of First Midwest with respect to such shares that is smaller than the stockholder’s current percentage ownership of NI Bancshares. Upon receiving 2.8858 shares of First Midwest common stock per issued and outstanding share of NI Bancshares common stock following the effective time of the merger, the former stockholders of NI Bancshares as a group would receive shares in the merger constituting approximately 3.8% of the outstanding shares of First Midwest common stock immediately after the merger based on the number of shares of First Midwest common stock outstanding as of January 14, 2016, the latest practicable date before the printing of this proxy statement/prospectus.  Because of this, NI Bancshares stockholders will have less influence on the management and policies of First Midwest than they now have on the management and policies of NI Bancshares.

The opinion of NI Bancshares’ financial advisor will not reflect changes in circumstances between the signing of the merger agreement and the completion of the merger.

NI Bancshares has not obtained an updated opinion from Sheshunoff, its financial advisor, as of the date of this proxy statement/prospectus.  Sheshunoff’s opinion was based on certain facts and assumptions regarding the operations and prospects of First Midwest and NI Bancshares, general market and economic conditions and other factors.  Changes in the operations and prospects of First Midwest or NI Bancshares, general market and economic conditions and other factors that may be beyond the control of First Midwest or NI Bancshares may significantly

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alter the value of First Midwest or NI Bancshares, the prices of the shares of First Midwest common stock by the time the merger is completed or the future price at which First Midwest common stock trades.  Sheshunoff’s opinion does not speak as of the time the merger will be completed or as of any date other than the date of such opinion.  The opinion will not address the fairness of the merger consideration from a financial point of view at the time a NI Bancshares stockholder votes or at the time the merger is completed.  However, NI Bancshares’ Board of Directors’ recommendation that NI Bancshares stockholders vote “FOR” adoption of the merger agreement is made as of the date of this proxy statement/prospectus.  For a description of the opinion that NI Bancshares received from Sheshunoff, please refer to “The Merger—Opinion of NI Bancshares’ Financial Advisor” on page 30.”

The shares of First Midwest common stock you will receive as a result of the merger will have different rights from your shares of NI Bancshares common stock.

The rights associated with NI Bancshares common stock are different from the rights associated with First Midwest common stock.  Please see “Comparison of Stockholder Rights” on page 70 for a discussion of the different rights associated with First Midwest common stock.

Under certain circumstances, the cash component of the merger consideration may be reduced due to environmental conditions and/or title defects on NI Bancshares’ real property.

Under certain circumstances, if certain environmental conditions and/or title defects exist with respect to NI Bancshares’ real property and the cost to remediate and/or cure such conditions or defects is greater than $150,000, the cash received by the holders of NI Bancshares common stock in connection with the merger will be reduced by such cost up to an aggregate amount of $1,000,000.   If such cost exceeds $1,000,000, First Midwest may terminate the merger agreement or reduce the cash component of the consideration by $1,000,000 and, subject to the satisfaction of all other conditions, proceed to the closing of the merger. If the merger is approved by the NI Bancshares stockholders, the cash consideration to be received by individual stockholders may be subject to this reduction without their further consent. See “The Merger Agreement—Merger Consideration” on page 44.

First Midwest will be subject to heightened regulatory requirements if it exceeds $10 billion in assets.

At September 30, 2015, First Midwest and First Midwest Bank had approximately $9.9 billion in total consolidated assets.  First Midwest and First Midwest Bank will exceed $10 billion in total consolidated assets as a result of the transactions contemplated by the merger agreement or in the future if they continue to grow. Although First Midwest has developed infrastructure to address the regulatory requirments described below and believes it is prepared to exceed $10 billion in total assets, exceeding $10 billion in total assets will subject First Midwest and First Midwest Bank to heightened regulatory requirements and costs, risks and uncertainties associated therewith.

The Dodd-Frank Wall Street Reform and Consumer Protection Act (the “Dodd-Frank Act”) and its implementing regulations impose various additional requirements on bank holding companies with $10 billion or more in total assets, including compliance with portions of the Federal Reserve’s enhanced prudential oversight requirements and annual stress testing requirements.  In addition, banks with $10 billion or more in total assets are primarily examined by the Consumer Financial Protection Bureau (the “CFPB”) with respect to various federal consumer financial protection laws and regulations.  As a relatively new agency with evolving regulations and practices, there is uncertainty as to how the CFPB’s examination and regulatory authority might impact First Midwest and First Midwest Bank’s business.

Compliance with these requirements may require First Midwest to hire additional compliance or other personnel, design and implement additional internal controls, or incur other significant expenses.  Compliance with the annual stress testing requirements, part of which must be publicly disclosed, may also be misinterpreted by the market generally or First Midwest’s customers and, as a result, may adversely affect First Midwest’s stock price or First Midwest’s ability to retain its customers or effectively compete for new business opportunities.  To ensure compliance with these heightened requirements when effective, First Midwest’s regulators may require it to fully comply with these requirements or take actions to prepare for compliance even before First Midwest’s or First Midwest Bank’s total assets equal or exceed $10 billion.  As a result, First Midwest may incur compliance-related costs before it might otherwise be required, including if First Midwest does not continue to grow at the rate it expects or at all.  First Midwest’s regulators may also consider its preparation for compliance with these regulatory requirements when examining its operations generally or considering any request for regulatory approval First

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Midwest may make, even requests for approvals on unrelated matters.  These factors could have a material adverse effect on First Midwest’s business, financial condition or results of operations.

Completion of the merger is subject to certain conditions, and if these conditions are not satisfied or waived, the merger will not be completed.

The obligations of First Midwest and NI Bancshares to complete the merger are subject to satisfaction or waiver (if permitted) of a number of conditions. The satisfaction of all of the required conditions could delay the completion of the merger for a significant period of time or prevent it from occurring. Any delay in completing the merger could cause the combined company not to realize some or all of the benefits that the combined company expects to achieve if the merger is successfully completed within its expected timeframe. Further, there can be no assurance that the conditions to the closing of the merger will be satisfied or waived or that the merger will be completed. See “The Merger Agreement—Conditions to Completion of the Merger” on page 53.

In addition, if the merger is not completed on or before November 12, 2016, either First Midwest or NI Bancshares may choose not to proceed with the merger. NI Bancshares may also terminate the merger agreement under certain circumstances  First Midwest may also terminate the merger agreement under certain circumstances. See “The Merger Agreement—Termination of the Merger Agreement” on page 55.

The financial forecasts reflected in the opinion of Sheshunoff, which is summarized beginning on page 30, involve risks, uncertainties and assumptions made by Sheshunoff, many of which are beyond the control of First Midwest and NI Bancshares. As a result, they may not prove to be accurate and are not necessarily indicative of current values or future performance of either First Midwest or NI Bancshares.

The financial forecasts of Sheshunoff reflected in its fairness opinion, a copy of which is attached to this proxy statement/prospectus as Appendix B, and which is summarized beginning on page 30, involve risks, uncertainties and assumptions made by Sheshunoff and are not a guarantee of future performance. The future financial results of First Midwest and NI Bancshares and, if the merger is completed, the combined company, may materially differ from those expressed in the financial forecasts of Sheshunoff due to factors that are beyond First Midwest’s and NI Bancshares’ ability to control or predict. Neither First Midwest nor NI Bancshares can provide any assurance that these financial forecasts will be realized or that First Midwest’s or NI Bancshares’ future financial results will not materially vary from such financial forecasts. First Midwest did not provide its own financial forecasts and the management of First Midwest did not confirm or otherwise comment with respect to any estimates used by or the financial forecasts of Sheshunoff, nor do First Midwest or NI Bancshares undertake to update the forecasts reflected in Sheshunoff’s fairness opinion. Such financial forecasts cover multiple years, and the information by its nature becomes subject to greater uncertainty with each successive year. These financial forecasts do not take into account any circumstances or events occurring after the date they were prepared.

More specifically, the financial forecasts of Sheshunoff:

·                  necessarily make numerous assumptions by Sheshunoff, many of which are beyond the control of First Midwest or NI Bancshares and may not prove to be accurate;

·                  do not necessarily reflect revised prospects for First Midwest’s or NI Bancshares’ businesses, changes in general business or economic conditions, or any other transaction or event that has occurred or that may occur and that was not anticipated at the time the forecasts were prepared;

·                  are not necessarily indicative of current values or future performance, which may be significantly more favorable or less favorable than is reflected in the forecasts; and

·                  should not be regarded as a representation that the financial forecasts will be achieved.

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The financial forecasts reflected in Sheshunoff’s fairness opinion were not prepared with a view toward public disclosure or compliance with published guidelines of the SEC or the American Institute of Certified Public Accountants for preparation and presentation of prospective financial information or GAAP, and do not reflect the effect of any proposed or other changes in GAAP that may be made in the future.

NI BANCSHARES SPECIAL MEETING

This section contains information from NI Bancshares for NI Bancshares stockholders about the special meeting NI Bancshares has called to consider and approve the merger agreement and the transactions contemplated thereby, including the merger.  We are mailing this proxy statement/prospectus to you, as a NI Bancshares stockholder, on or about January 22, 2016.  Together with this proxy statement/prospectus, we are also sending to you a notice of the NI Bancshares special meeting and a form of proxy card that NI Bancshares’ board of directors is soliciting for use at the special meeting of NI Bancshares stockholders and at any adjournments of the meeting.

This proxy statement/prospectus is also being furnished by First Midwest to NI Bancshares stockholders as a prospectus in connection with the issuance of shares of First Midwest common stock upon completion of the merger.

Date, Time and Place

The special meeting of NI Bancshares stockholders will be held on February 25, 2016 , at 10:30 a.m.  Central Time, at DeKalb County Farm Bureau, located at 1350 W. Prairie Drive, Sycamore, Illinois, 60178.

Matters To Be Considered

At the special meeting, NI Bancshares stockholders as of the record date will be asked to consider and vote on the following matters:

·                  To approve and adopt the merger agreement, pursuant to which NI Bancshares will merge with and into First Midwest, with First Midwest as the surviving entity of the merger, and to approve and adopt the other transactions contemplated thereby, including the merger.

·                  To approve one or more adjournments of the special meeting, if necessary or appropriate, including adjournments to permit further solicitation of proxies in favor of approving and adopting the merger agreement and the other transactions contemplated thereby, including the merger (which we refer to as the “adjournment proposal”).

·                  To act upon such other business as may properly come before the special meeting and any adjournments thereof.

Recommendation of NI Bancshares’ Board

NI Bancshares’ board of directors (i) believes that the merger agreement and the transactions contemplated thereby, including the merger, are in the best interests of NI Bancshares and its stockholders and that the merger consideration is fair to NI Bancshares stockholders, (ii) has unanimously approved and adopted the merger agreement and (iii) unanimously recommends that NI Bancshares stockholders vote “FOR” approval and adoption of the merger agreement and the transactions contemplated thereby, including the merger, and “FOR” the adjournment proposal.  See “The Merger—NI Bancshares’ Reasons for the Merger and Recommendations of the Board of NI Bancshares” on page 29 for a more detailed discussion of NI Bancshares’ board of directors’ recommendation with regard to the merger agreement and the transactions contemplated thereby, including the merger.

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Record Date and Quorum

NI Bancshares’ board of directors has fixed the close of business on January 11, 2016 as the record date for determining the NI Bancshares stockholders entitled to receive notice of and to vote at the special meeting.  NI Bancshares stockholders are entitled to one vote on each matter considered and voted on at the special meeting for each share of NI Bancshares common stock held of record at the close of business on the record date.  Only NI Bancshares stockholders of record as of the record date are entitled to notice of and to vote at the special meeting and any adjournments of the special meeting.

As of the record date, 1,057,815 shares of NI Bancshares common stock were issued and outstanding and held by approximately 312 record holders.  The presence at the special meeting, in person or by proxy, of holders of a majority of the outstanding shares of NI Bancshares common stock entitled to vote at the special meeting will constitute a quorum for the transaction of business. All shares of NI Bancshares common stock present in person or represented by proxy, including abstentions, will be treated as present for purposes of determining the presence or absence of a quorum for all matters voted on at the special meeting but shares represented by a proxy from a broker, bank or nominee indicating that such person has not received instructions from the beneficial owner or other person entitled to vote the shares, which we refer to as “broker non-votes,” will not be counted as shares present.

Vote Required; Treatment of Abstentions and Failure to Vote

Approval of the merger agreement and the transactions contemplated thereby, including the merger, requires the affirmative vote of NI Bancshares stockholders representing a majority of the outstanding shares of NI Bancshares common stock as of the record date.  The merger agreement and the completion of the transactions contemplated thereby, including the merger, will not require the approval of the holders of First Midwest common stock under the DGCL or applicable rules of the NASDAQ Stock Market.  If you fail to submit a proxy card or vote in person at the NI Bancshares special meeting, mark “ABSTAIN” on your proxy card or fail to instruct your bank or broker with respect to the proposal to approve the merger agreement and the transactions contemplated thereby, including the merger, it will have the same effect as a vote “AGAINST” approval of the merger agreement and the transactions contemplated thereby, including the merger.

Approval of the adjournment proposal requires the affirmative vote of a majority of votes cast at the special meeting.  If you mark “ABSTAIN” with respect to the adjournment proposal, it will have the same effect as a vote “AGAINST” the adjournment proposal.  If you fail to submit a proxy card or vote in person at the special meeting or fail to instruct your bank or broker with respect to the adjournment proposal, it will have no effect on such proposal.

Shares Held by Directors and Officers

As an inducement to and condition of First Midwest’s willingness to enter into the merger agreement, all of the directors and certain officers of NI Bancshares and NB&T who beneficially owned in the aggregate approximately 21% of NI Bancshares’ outstanding common stock as of November 12, 2015 entered into voting agreements, pursuant to which, among other things, they agreed to vote all of their shares of NI Bancshares common stock in favor of approval of the merger agreement and the transactions contemplated thereby, including the merger, other matters required to be approved or adopted to effect the merger and any other transactions contemplated by the merger agreement.  See “The Merger—Interests of Certain Persons in the Merger”.

As of the record date, First Midwest and its subsidiaries held no shares of NI Bancshares common stock and none of its directors and executive officers or their affiliates held any shares of NI Bancshares common stock.

Participants in the NI Bancshares Stock Plans

NI Bancshares sponsors the NI Bancshares Stock Plans.  Per the terms of the NI Bancshares Stock Plans, and award agreements thereunder, holders of stock options do not have voting rights with respect to the shares

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underlying unexercised stock options, whereas recipients of restricted stock awards are entitled to vote on behalf of underlying shares, even while subject to vesting requirements, from the date of grant until such awards are forfeited.  Holders of NI Bancshares restricted stock will vote in the same manner as holders of NI Bancshares common stock, as discussed above.  As of the record date, participants in the NI Bancshares Stock Plans held approximately 23% of the shares entitled to vote at the  special meeting.

Solicitation of Proxies; Payment of Solicitation Expenses

Proxies are being solicited by NI Bancshares’ board of directors from NI Bancshares stockholders.  Shares of NI Bancshares common stock represented by properly executed proxies, and that have not been revoked, will be voted in accordance with the instructions indicated on the proxies.  If no instructions are indicated, such proxies will be voted “FOR” approval and adoption of the merger agreement and the transactions contemplated thereby, including the merger, “FOR” the adjournment proposal, and in the discretion of the individuals named as proxies as to any other matter that may come before the special meeting.

First Midwest and NI Bancshares have agreed to each pay for one-half of the costs and expenses (excluding the fees and disbursements of counsel, financial advisors and accountants) of copying, printing and distributing this proxy statement/prospectus and all listing, filing or registration fees, including fees paid for filing the registration statement of which this proxy statement/prospectus is a part with the SEC and any other fees paid for filings with governmental authorities.  In addition to the solicitation of proxies by mail, solicitation may be made by certain directors, officers or employees of NI Bancshares or its affiliates telephonically, electronically or by other means of communication.  Directors, officers and employees will receive no additional compensation for such solicitation.   NI Bancshares will reimburse brokers and other nominees for costs incurred by them in mailing proxy materials to beneficial owners in accordance with applicable rules.

Voting Your Shares via Mail

You may vote your proxy by mailing your Proxy Card to American Stock Transfer & Trust Company, NI Bancshares’ transfer agent. Please complete, sign, date and return the enclosed proxy card in the postage-paid envelope provided to American Stock Transfer & Trust Company, Attn: Proxy Dept., 6201 15th Avenue, Brooklyn, NY 11219, before 5:00 p.m. Eastern Time on February 24, 2016.

In order to be effective, proxy instructions must be received before the time indicated above to allow for processing the results.

Revocability of Proxies and Changes to a NI Bancshares Stockholder’s Vote

A NI Bancshares stockholder who has given a proxy may revoke it at any time before its exercise at the special meeting by (i) giving a written notice of revocation to James W. Dutton, Chairman of the Board of NI Bancshares, (ii) attending the special meeting in person and voting by ballot at the special meeting, or (iii) by properly submitting to NI Bancshares a duly executed proxy bearing a later date.  All written notices of revocation and other communications with respect to revocation of proxies should be addressed to NI Bancshares as follows: 230 West State Street, Sycamore, Illinois 60178, Attention:  James W. Dutton, Chairman of the Board.

Attending the Meeting

All NI Bancshares stockholders, including stockholders of record and stockholders who hold their shares through brokers, trusts, banks, nominees, the NI Bancshares Stock Plans or any other holder of record, are invited to attend the special meeting.  Stockholders of record can vote in person at the special meeting.  If you are not a stockholder of record, you must obtain a proxy executed in your favor from the record holder of your shares, such as a broker, bank or other nominee, to be able to vote in person at the special meeting.  If you plan to attend the special meeting, you must hold your shares in your own name or have a letter from the record holder of your shares confirming your ownership. In addition, you must bring a form of personal photo identification with you in order to be admitted.  NI Bancshares reserves the right to refuse admittance to anyone without proper proof of share ownership and without proper photo identification.

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THE MERGER

The following discussion describes certain material information about the merger.  We urge you to read carefully this entire document, including the merger agreement and the financial advisor opinion attached as Appendices A and B, respectively, to this proxy statement/prospectus, for a more complete understanding of the merger.

NI Bancshares’ board of directors has unanimously approved and adopted the merger agreement and the transactions contemplated thereby, including the merger.  The merger agreement provides for combining our companies through the merger of NI Bancshares with and into First Midwest, with First Midwest as the surviving corporation.  As a result, the separate existence of NI Bancshares will terminate.  Following the merger at a time yet to be determined, NB&T, NI Bancshares’ wholly owned bank subsidiary, will merge with and into First Midwest Bank, First Midwest’s wholly owned bank subsidiary.  We expect to complete the mergers in the first quarter of 2016, although delays may occur.

Upon completion of the merger, NI Bancshares stockholders will receive for each share of NI Bancshares common stock that they own immediately prior to the completion of the merger 2.8858 fully paid and non-assessable shares of First Midwest common stock plus $13.17 in cash, without interest. However, if certain environmental conditions and/or title defects exist with respect to NI Bancshares’ real property and the cost to remediate and/or cure such conditions or defects is greater than $150,000, the cash received by the holders of NI Bancshares common stock in connection with the merger will be reduced by such cost up to an aggregate amount of $1,000,000.  If such cost exceeds $1,000,000, First Midwest may terminate the merger agreement or reduce the cash component of the consideration by $1,000,000 and, subject to the satisfaction of all other conditions, proceed to the closing of the merger.  In addition, NI Bancshares will have a right to terminate the merger agreement if the volume weighted average closing price of First Midwest common stock during a specified period before the effective time of the merger both (i) is less than $14.6055 per share and (ii) underperforms a specified index of financial institution stocks during such period by more than 15%. However, NI Bancshares would not have the right to terminate the merger agreement under these circumstances if First Midwest were to elect to increase the exchange ratio or the amount of cash to be received per share of NI Bancshares common stock as provided in the merger agreement.  If this election is made, NI Bancshares may not terminate the merger agreement under these circumstances.

Upon completion of the merger, each outstanding stock option (vested or unvested) to purchase NI Bancshares common stock will be cancelled and terminated in exchange for the right to receive cash, without any interest and subject to any required withholding tax, in an amount equal to the merger consideration value per share, as defined in the merger agreement, minus the applicable exercise price of such outstanding stock option, multiplied by the number of shares of NI Bancshares common stock covered by such outstanding stock option.  Any out-of-the money stock options granted by NI Bancshares will be cancelled and no consideration will be provided.  An out-of-the-money stock option is an outstanding stock option for which the exercise price per share is greater than or equal to the merger consideration value per share described above.  The merger consideration value per share means the sum of (i) the product of (a) 2.8858 multiplied by (b) the price of First Midwest common stock on the trading day immediately prior to the effective time of the merger, as provided in the merger agreement, plus (ii) $13.17.

Upon completion of the merger, each outstanding unvested restricted stock award will become fully vested and the shares of NI Bancshares covered by such award shall be exchanged for the same merger consideration as all other shares of NI Bancshares common stock.  Shares of NI Bancshares common stock received upon vesting of restricted stock awards or exercise of stock options and held immediately prior to the effective time of the merger will also receive the merger consideration.

Shares of First Midwest common stock issued and outstanding at the completion of the merger will remain outstanding and those shares of common stock will be unaffected by the merger.  First Midwest common stock will

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continue to trade on the NASDAQ Stock Market following the merger under the First Midwest Bancorp, Inc. name with the symbol “FMBI.”

Please see “The Merger Agreement” for additional and more detailed information regarding the legal documents that govern the merger, including information about the conditions to the merger and the provisions for terminating or amending the merger agreement.

Background of the Merger

NI Bancshares’ board of directors and senior management regularly review and evaluate NI Bancshares’ business, strategic direction, performance, prospects and strategic alternatives.  In early 2015, NI Bancshares’ senior management had discussions with representatives of Sheshunoff, an investment banking firm, regarding NI Bancshares’ strategic alternatives, including a possible “merger of equals”, a sale of the organization or growing the organization either organically or through acquisitions.  Representatives of Barack Ferrazzano, NI Bancshares’ special counsel, also met with members of management and the board of directors to discuss the legal and fiduciary implications of NI Bancshares’ exploration of strategic alternatives. At a meeting of the board of directors in February, 2015, Barack Ferrazzano gave a comprehensive presentation concerning the fiduciary duties of the board of directors when considering strategic alternatives and engaged in extensive discussion with the directors regarding these duties.

Over the next several weeks, NI Bancshares’ senior management and representatives from Sheshunoff discussed the advantages and disadvantages of remaining an independent financial institution, the historical performance and strategic direction of NI Bancshares, increased competition in NB&T’s market area, anticipated increases in operating and regulatory costs and capital requirements and the lack of liquidity in NI Bancshares’ common stock.  In addition, they discussed the range of possible valuations for a sale of NI Bancshares and potential transaction partners.  As part of this discussion, NI Bancshares’ senior management considered the increasing amount of competition in the communities that NB&T serves and within the greater Chicago metropolitan area, the continuing low interest rate environment and trends in mergers and acquisitions in the financial services sector.

Following these discussions, NI Bancshares’ senior management asked Sheshunoff to prepare a presentation to discuss strategic alternatives with the board of directors.  NI Bancshares’ board of directors held a meeting in March, 2015, and reviewed with Sheshunoff its presentation and evaluated the current banking environment, financial services industry trends, merger and acquisition activity within the industry, trends in pricing of bank transactions and NI Bancshares’ strategic alternatives, including, but not limited to, a possible merger with or sale to a larger institution.  Sheshunoff discussed with NI Bancshares’ board the potential pricing NI Bancshares might anticipate should it decide to consider a possible sale transaction. Sheshunoff also identified several financial institutions, including First Midwest, that could have interest in a possible business combination with NI Bancshares and provided the board of directors with a potential timeline for any such transaction.

Following this financial presentation, NI Bancshares’ board engaged in a detailed discussion regarding the prospects of the organization in light of the risks and uncertainties related to increased competition in NB&T’s market area, anticipated increases in operating and regulatory costs and capital requirements and the continuing low interest rate environment, and the possible strategic alternatives discussed with Sheshunoff.  At a meeting of the board of directors held on May 21, 2015, NI Bancshares’ board authorized management to investigate possible strategic transactions and approved the engagement of Sheshunoff as its financial advisor. NI Bancshares’ board also formed a capital committee comprised of independent directors to gather relevant information regarding any potential strategic transaction and to prepare and present to the board of directors any proposals with respect thereto.  NI Bancshares’ board instructed Sheshunoff to continue evaluating the market and identifying possible transaction partners.  NI Bancshares and Sheshunoff entered into a formal engagement letter on May 21, 2015.

Sheshunoff then began a comprehensive due diligence review of NI Bancshares and NB&T, meeting with members of NI Bancshares’ management, and began developing confidential marketing materials concerning NI Bancshares. Sheshunoff also performed due diligence regarding potential acquisition transaction partners.

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Representatives of Barack Ferrazzano met with members of the NI Bancshares management team and the capital committee to discuss certain employee benefits matters, including the formation of a bank-wide severance plan. On June 18, 2015, Sheshunoff gave a financial presentation to the board of directors which included, among other matters, a list of prospective acquirers divided into two tiers based on the likelihood, in Sheshunoff’s judgment, of being a suitable strategic partner. After the board of directors and the capital committee gave their input to this bifurcated list and the offering memorandum, Sheshunoff began contacting prospective bidders and distributed confidentiality agreements to those bidders included on the “Tier 1” list. Sheshunoff provided copies of the confidential marketing materials to each party that had executed a confidentiality agreement. On July 7, 2015, representatives of Sheshunoff approached First Midwest to notify it of the auction to be conducted for NI Bancshares and, on July 15, 2015, First Midwest entered into a confidentiality agreement with NI Bancshares and Sheshunoff.  First Midwest engaged Sandler O’Neill + Partners (“Sandler O’Neill”) to advise First Midwest on the potential acquisition of NI Bancshares.

At a meeting of NI Bancshares’ board on July 16, 2015, Sheshunoff reviewed the results of the sale process with NI Bancshares’ board and management.  Sheshunoff reported that it had contacted twenty prospective strategic partners, eight of which had executed confidentiality agreements and received copies of the confidential marketing materials and access to an electronic data room.  Sheshunoff continued working with potential strategic partners, including First Midwest, with the goal of receiving initial indications of interest by late July or early August. By July 22, 2015, a total of 11 strategic partners had executed confidentiality agreements and received copies of the confidential marketing materials and access to an electronic data room.

On August 7, 2015, First Midwest submitted a preliminary non-binding indication of interest for NI Bancshares.  At a meeting of the capital committee held on August 12, 2015, Sheshunoff presented an in-depth report regarding First Midwest’s indication of interest for a proposed acquisition with First Midwest, subject to completion of its due diligence and the negotiation of a definitive agreement.  Sheshunoff and the capital committee discussed the price range of the proposal received, the form of consideration offered, the reputation of First Midwest, the strategic opportunity offered by the possible transaction and the perceived ability of First Midwest to complete a transaction.  The board members asked Sheshunoff a number of questions regarding the indication of interest and its terms.  They also engaged in a long discussion among themselves regarding the advisability of proceeding with any type of strategic transaction, and if so, the relative advantages and disadvantages of the indication of interest.  They also discussed whether to contact additional potential strategic partners. On August 20, 2015, NI Bancshares’ board of directors met to review recommendations of the capital committee regarding the indication of interest submitted by First Midwest, including the terms of a counter-proposal. The board of directors authorized Sheshunoff to communicate these certain terms to First Midwest and, that same day, First Midwest and representatives from Sandler O’Neill held discussions with NI Bancshares’ representatives from Sheshunoff to discuss NI Bancshares’ potential counter proposal.

On September 24, 2015, members of the senior management of First Midwest and NI Bancshares, along with representatives of Sheshunoff and Sandler O’Neill, met at First Midwest’s headquarters in order for First Midwest to conduct interviews with members of NI Bancshares’ management, and to allow First Midwest to hold discussions with Sheshunoff regarding interim financial results and analyst earnings expectations for 2015 and 2016.

At about this time, NI Bancshares’ Chief Executive Officer was approached by a financial institution that indicated it was interested in exploring a possible transaction with NI Bancshares. After consultation with the board of directors and entry into a confidentiality agreement, Sheshunoff provided the institution with the confidential marketing materials and allowed the institution to conduct due diligence to the same degree as First Midwest. The institution presented NI Bancshares with a preliminary indication of interest in early October.  While NI Bancshares considered this preliminary indication of interest, First Midwest continued its due diligence process.

After receiving an updated indication of interest from First Midwest on October 9, 2015, Sheshunoff engaged in discussions with both First Midwest and the second financial institution to clarify elements of their proposals, including with respect to the nature and amount of the financial consideration.  Thereafter, the board again met with its legal and financial representatives to review the indications of interest and to compare the opportunities offered by the possible transactions with First Midwest and the other financial institution. The board ultimately concluded that, given the relative merits of the proposals presented by Sheshunoff, the indication of

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interest from First Midwest was the more attractive proposal received, as the other indication of interest included a lower amount of consideration and other less desirable terms, including timing, certainty of transaction execution and liquidity aspects.  The board determined that NI Bancshares should continue to move forward with First Midwest, allowing First Midwest to conduct additional due diligence in order to obtain a final bid.

On October 15, 2015, NI Bancshares directed Sheshunoff to submit a counter proposal as to valuation and certain other terms of a transaction and, that same day, First Midwest communicated a counter proposal to Sheshunoff, through Sandler O’Neill, as to valuation and certain other terms of a transaction.

On October 16, 2015, NI Bancshares directed Sheshunoff to communicate its acceptance of First Midwest’s final offer of approximately $71 million consisting of cash and stock of First Midwest with a thirty-day exclusivity period to negotiate and publicly announce a definitive agreement.

On October 21, 2015, First Midwest and NI Bancshares entered into an exclusivity agreement.  This allowed First Midwest to conduct its due diligence through November, 2015.  Throughout the due diligence process, Sheshunoff remained in contact with First Midwest to assist in the due diligence process, including by providing requested information and by participating in the negotiation of the terms of the merger agreement.

On October 23, 2015, NI Bancshares and Barack Ferrazzano received an initial draft of the merger agreement from Chapman and Cutler, First Midwest’s outside counsel.  From that time until November 12, 2015, First Midwest continued its due diligence process and NI Bancshares, Sheshunoff and Barack Ferrazzano negotiated the terms of the merger agreement and related documents with First Midwest, Chapman and Cutler and Sandler O’Neill.

The boards of directors of NI Bancshares and NB&T each held a special meeting on November 9, 2015.  Representatives of Barack Ferrazzano and Sheshunoff reviewed with the boards of directors the process leading to the proposed transaction and the course of negotiations with First Midwest.  Representatives of Barack Ferrazzano reviewed with the boards of directors the terms of the then current draft of the merger agreement including the scope of the representations and warranties, the nature of NI Bancshares’ operating covenants prior to closing and the proposed closing conditions and termination provisions, including an explanation regarding the break-up fee provisions.  Sheshunoff provided to the boards of directors a financial analysis of the proposed transactions with First Midwest and reviewed in detail with the boards the terms of the merger consideration, including a discussion of the floor and cap to the exchange ratio.  At that meeting, Sheshunoff provided its oral and written opinion that the financial terms of the Agreement were fair to NI Bancshares and its stockholders from a financial point of view.  The boards engaged in a long discussion amongst themselves and with NI Bancshares’ advisors regarding the proposed draft of the merger agreement, including the final business terms of the transaction.

After the conclusion of the presentations and discussions at the November 9, 2015 meeting, NI Bancshares’ board of directors unanimously approved the merger agreement, the merger and related documents, adopted resolutions recommending that NI Bancshares stockholders approve and adopt the merger agreement and the merger, and authorized the NI Bancshares officers to execute the merger agreement on behalf of NI Bancshares in substantially the form reviewed by the board subject to such changes agreed to by the authorized officers of NI Bancshares.  On the same day, the NB&T board of directors unanimously approved the merger of the bank with and into First Midwest Bank and recommended to its sole stockholder, NI Bancshares, that it approve the merger, which was subsequently approved by NI Bancshares.

After the conclusion of the presentations and discussions at the November 10, 2015 meeting, First Midwest’s board of directors unanimously approved the merger agreement, the merger and related documents, and authorized the First Midwest officers to execute the merger agreement on behalf of First Midwest in substantially the form reviewed by the board subject to such changes agreed to by the authorized officers of First Midwest.  On the same day, the First Midwest Bank board of directors unanimously approved the merger of the bank with and into

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NB&T and recommended to its sole stockholder, First Midwest, that it approve the merger, which was subsequently approved by First Midwest.

On November 12, 2015, the merger agreement and related documents were finalized and executed by NI Bancshares and First Midwest.  First Midwest issued a press release the morning of November 12, 2015, announcing the execution of the merger agreement.

First Midwest’s Reasons for the Merger

First Midwest’s board of directors believes that the merger is in the best interests of First Midwest and its stockholders. In deciding to approve the merger, First Midwest’s board of directors considered a number of factors, including:

·                  management’s view that the acquisition of NI Bancshares provides an attractive opportunity to expand into desirable markets in DeKalb County, Kane County and LaSalle County, Illinois, and surrounding areas, as well as significantly expand First Midwest’s wealth management business;

·                  NI Bancshares’ community banking orientation and its compatibility with First Midwest and its subsidiaries;

·                  a review of the markets in which NI Bancshares operates;

·                  management’s review of the business, operations, earnings and financial condition, including capital levels and asset quality, of NB&T;

·                  projected efficiencies to come from integrating certain of NI Bancshares’ operations into First Midwest’s existing operations; and

·                  the likelihood that the merger will be approved by the relevant bank regulatory authorities without undue burden and in a timely manner.

The above discussion of the information and factors considered by First Midwest’s board of directors is not intended to be exhaustive, but includes a description of all material factors considered by First Midwest’s board. First Midwest’s board of directors further considered various risks and uncertainties related to each of these factors and the ability to complete the merger. In view of the wide variety of factors considered by First Midwest’s board of directors in connection with its evaluation of the merger, First Midwest’ board did not consider it practical to, nor did it attempt to, quantify, rank or otherwise assign relative weights to the specific factors that it considered.  In considering the factors described above, individual directors may have given differing weights to different factors. First Midwest’s board of directors collectively made its determination with respect to the merger based on the conclusion reached by its members, based on the factors that each of them considered appropriate, that the merger is in the best interests of First Midwest stockholders and that the benefits expected to be achieved from the merger outweigh the potential risks and vulnerabilities.

NI Bancshares’ Reasons for the Merger and Recommendations of the Board of NI Bancshares

NI Bancshares’ board of directors has concluded that the merger offers NI Bancshares stockholders an attractive opportunity to achieve the board’s strategic business objectives, including increasing stockholder value,

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growing the size of the business and enhancing liquidity for NI Bancshares stockholders. In addition, NI Bancshares’ board of directors believes that the customers and communities served by NB&T will benefit from the merger.

In deciding to approve the merger agreement and the transactions contemplated thereby, NI Bancshares’ board of directors consulted with NI Bancshares’ management, as well as its legal counsel, Barack Ferrazzano, and financial advisor, Sheshunoff, and considered numerous factors, including the following:

·                  information with respect to the businesses, earnings, operations, financial condition, prospects, capital levels and asset quality of NI Bancshares and First Midwest, both individually and as a combined company;

·                  the value to be received by NI Bancshares stockholders in the merger as compared to stockholder value projected for NI Bancshares as an independent entity;

·                  the enhanced liquidity for NI Bancshares stockholders, including with respect to the First Midwest common stock to be received in the merger;

·                  the compatibility of NI Bancshares’ geographical footprint with that of First Midwest and the expansion of product and service availability to the communities currently served by NI Bancshares;

·                  the market value of First Midwest common stock prior to the execution of the merger agreement and the prospects for future appreciation in the stock;

·                  the perceived risks and uncertainties attendant to NI Bancshares’ operation as an independent banking organization, including the risks and uncertainties related to the continuing low-interest rate environment, competition in NI Bancshares’ market area, increased operating and regulatory costs and increased capital requirements;

·                  the NI Bancshares’ board of directors’ belief that combining with a larger financial institution will benefit stockholders and customers in that the combined organization will be better equipped to respond to economic and industry developments and should be better positioned to develop and build on its position in existing markets;

·                  the fact that First Midwest is publicly held and the merger would provide access to a public trading market for NI Bancshares stockholders whose investment is currently in a company with a very limited trading market;

·                  the effects of the merger on NI Bancshares’ employees, including the retention of a significant number of employees and their ability to participate in First Midwest’s benefit plans;

·                  the status of First Midwest as a regular participant in the mergers and acquisitions market within the banking industry and the likelihood that its sophistication in such matters would result in greater execution certainty and provide for efficiencies in connection therewith; and

·                  the likelihood that the merger will be approved by the relevant bank regulatory authorities without undue burden and in a timely manner.

The above discussion of the information and factors considered by NI Bancshares’ board of directors is not intended to be exhaustive, but includes a description of all material factors considered by NI Bancshares’ board.  NI Bancshares’ board of directors further considered various risks and uncertainties related to each of these factors and the ability to complete the merger.  In view of the wide variety of factors considered by NI Bancshares’ board of directors in connection with its evaluation of the merger, NI Bancshares’ board did not consider it practical to, nor did it attempt to, quantify, rank or otherwise assign relative weights to the specific factors that it considered.  In considering the factors described above, individual directors may have given differing weights to different factors. NI Bancshares’ board of directors collectively made its determination with respect to the merger based on the conclusion reached by its members, based on the factors that each of them considered appropriate, that the merger is

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in the best interests of NI Bancshares stockholders and that the benefits expected to be achieved from the merger outweigh the potential risks and vulnerabilities.

After considering the foregoing and other relevant factors and risks, and their overall impact on the stockholders and other constituencies of NI Bancshares, NI Bancshares’ board of directors concluded that the anticipated benefits of the merger outweighed the anticipated risks of the transaction.  Accordingly, NI Bancshares’ board of directors unanimously approved the merger agreement and the merger, and the board of directors unanimously recommends that NI Bancshares stockholders vote FOR the proposal to approve and adopt the merger agreement and the merger.

Opinion of NI Bancshares’ Financial Advisor

NI Bancshares engaged Sheshunoff to provide an opinion as to the fairness from a financial viewpoint to NI Bancshares stockholders of the merger consideration to be received by the stockholders of NI Bancshares.  As part of its investment banking business, Sheshunoff is regularly engaged in the valuation of securities in connection with mergers and acquisitions and valuations for tax, financial reporting, corporate and other purposes.  NI Bancshares retained Sheshunoff based upon its experience as a financial advisor in mergers and acquisitions of financial institutions and its knowledge of financial institutions.

On November 9, 2015, Sheshunoff rendered its fairness opinion to the board of directors of NI Bancshares that, as of such date, the merger consideration was fair, from a financial point of view, to the stockholders of NI Bancshares.  The full text of the fairness opinion which sets forth, among other things, assumptions made, procedures followed, matters considered, and limitations on the review undertaken, is attached as Appendix B to this proxy statement/prospectus.  The description of the opinion set forth below is qualified in its entirety by reference to the full text of the opinion. You are urged to read Sheshunoff’s fairness opinion carefully and in its entirety.  The fairness opinion is addressed to the board of directors of NI Bancshares and does not constitute a recommendation to any stockholder of NI Bancshares as to how he, she or it should vote at the special meeting of the stockholders of NI Bancshares.

Sheshunoff acted as financial advisor to NI Bancshares in connection with the proposed merger with First Midwest and participated in the negotiations leading to the merger agreement.  As part of its engagement, Sheshunoff assessed the fairness, from a financial point of view, of the merger consideration being received by the stockholders of NI Bancshares.  Sheshunoff attended the October 15, 2015 meeting at which NI Bancshares’ board of directors considered the merger agreement.  At that meeting, Sheshunoff presented its financial analysis of the transaction.  On November 9, 2015, Sheshunoff delivered to the board of directors its oral opinion, subsequently provided in final written form that same day, that the terms of the merger agreement were fair to NI Bancshares, and its stockholders, from a financial point of view.  Sheshunoff prepared the fairness opinion in connection with its general engagement to advise NI Bancshares in its exploration of strategic alternatives as further described in “—Background of the Merger.”

You should consider the following when reading the discussion of Sheshunoff’s opinion in this proxy statement/prospectus:

·                  The opinion letter details the procedures followed, assumptions made, matters considered, and qualifications and limitations of the review undertaken by Sheshunoff in connection with its opinion, and should be read in its entirety;

·                  Sheshunoff expressed no opinion as to the price at which shares of NI Bancshares or First Midwest common stock would actually be trading at any given time;

·                  Sheshunoff’s opinion does not address the relative merits of the merger and the other business strategies considered by NI Bancshares’ board of directors, nor does it address the board of directors’ decision to proceed with the merger; and

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·                  Sheshunoff’s opinion rendered in connection with the merger does not constitute a recommendation to any NI Bancshares stockholder as to how he or she should vote at the special meeting.

In connection with the fairness opinion, Sheshunoff:

·                  Reviewed the latest draft of the merger agreement;

·                  Discussed the terms of the merger agreement with the management of NI Bancshares and NI Bancshares’ legal counsel;

·                  Conducted conversations with the management of NI Bancshares regarding the recent and projected financial performance of NI Bancshares;

·                  Evaluated the financial condition of NI Bancshares based upon a review of regulatory reports for the five-year period ended December 31, 2014 and interim period through September 30, 2015, and internally-prepared financial reports for NI Bancshares for the interim period through September 30, 2015;

·                  Compared NI Bancshares’ recent operating results with those of certain other banks in the United States and the Midwest Region of the United States, as defined by SNL Financial, that have recently been acquired;

·                  Compared the pricing multiples for NI Bancshares in the merger to recent acquisitions of banks in the United States and the Midwest Region of the United States, as defined by SNL Financial, with similar characteristics to NI Bancshares;

·                  Analyzed the present value of the after-tax cash flows based on projections on a stand-alone basis approved by NI Bancshares through the five-year period ending September 30, 2020;

·                  Reviewed the potential pro forma impact of the merger on the combined company’s results and certain financial performance measures of NI Bancshares and First Midwest;

·                  Discussed certain matters regarding First Midwest’s regulatory standing, financial performance, and business prospects with First Midwest officers and representatives;

·                  Reviewed certain internal and publicly available information regarding First Midwest that Sheshunoff deemed relevant;

·                  Compared First Midwest’s recent operating results and pricing multiples with those of certain other publicly traded banks in the Midwest Region as defined by SNL financial that Sheshunoff deemed relevant;

·                  Reviewed available stock analyst research reports concerning First Midwest;

·                  Compared the historical stock price data and trading volume of First Midwest to certain relevant indices; and

·                  Performed such other analyses deemed appropriate.

For the purposes of the fairness opinion, Sheshunoff assumed and relied upon, without independent verification, the accuracy and completeness of the information provided to it by NI Bancshares in conjunction with the fairness opinion.  Sheshunoff assumed that any projections provided by or approved by NI Bancshares were

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reasonably prepared on a basis reflecting the best currently available estimates and judgments of NI Bancshares’ management.  Sheshunoff assumed such forecasts and projections will be realized in the amounts and at times contemplated thereby.  Sheshunoff assumes no responsibility for and expresses no opinion on any such projections or the assumptions on which they are based. In addition, where appropriate, Sheshunoff relied upon publicly available information that is believed to be reliable, accurate, and complete; however, Sheshunoff cannot guarantee the reliability, accuracy, or completeness of any such publicly available information.

Sheshunoff did not make an independent evaluation of the assets or liabilities (including any contingent, derivative or off-balance-sheet assets or liabilities) of NI Bancshares or First Midwest nor was Sheshunoff furnished with any such appraisal.  Sheshunoff assumed that any off-balance-sheet activities of NI Bancshares or First Midwest will not materially and adversely impact the future financial position or results of operations of First Midwest after the merger.  Sheshunoff is not an expert in the evaluation of loan portfolios for the purposes of assessing the adequacy of the allowance for loan and lease losses and assumed that such allowances for NI Bancshares and First Midwest are, respectively, adequate to cover such losses.

Sheshunoff assumed that the merger agreement, as provided to Sheshunoff, will be without any amendment or waiver of, or delay in the fulfillment of, any terms or conditions set for in the terms provided to Sheshunoff or any subsequent development that would have a material adverse effect on NI Bancshares or First Midwest and thereby on the results of its analyses.  Sheshunoff assumed that any and all regulatory approvals, if required, will be received in a timely fashion and without any conditions or requirements that could adversely affect the operations or financial condition of First Midwest after the completion of the merger.

The fairness opinion is necessarily based on economic, market, regulatory, and other conditions as in effect on, and the information made available to Sheshunoff as of November 9, 2015.

In rendering the fairness opinion, Sheshunoff performed a variety of financial analyses.  The preparation of an opinion involves various determinations as to the most appropriate and relevant methods of financial analysis and the application of those methods to the particular circumstances.  Consequently, the fairness opinion is not readily susceptible to partial analysis or summary description.  Moreover, the evaluation of fairness, from a financial point of view, of the merger consideration is to some extent subjective, based on the experience and judgment of Sheshunoff, and not merely the result of mathematical analysis of financial data.  Sheshunoff did not attribute particular weight to any analysis or factor considered by it.  Accordingly, notwithstanding the separate factors summarized below, Sheshunoff believes that its analyses must be considered as a whole and that selecting portions of its analyses and of the factors considered, without considering all analyses and factors, could create an incomplete view of the evaluation process underlying its opinion.  The ranges of valuations resulting from any particular analysis described below should not be taken to be Sheshunoff’s view of the actual value of NI Bancshares, First Midwest or the combined entity.

In performing its analyses, Sheshunoff made numerous assumptions with respect to industry performance, business and economic conditions and other matters, many of which are beyond the control of NI Bancshares or First Midwest.  The analyses performed by Sheshunoff are not necessarily indicative of actual values or future results, which may be significantly more or less favorable than suggested by such analyses.  In addition, Sheshunoff’s analyses should not be viewed as determinative of the opinion of the board of directors or the management of NI Bancshares with respect to the value of NI Bancshares or First Midwest or to the fairness of the merger consideration.

The following is a summary of the analyses performed by Sheshunoff in connection with its opinion.  The discussion utilizes financial information concerning NI Bancshares and First Midwest as of September 30, 2015 that is believed to be reliable, accurate, and complete; however, Sheshunoff cannot guarantee the reliability, accuracy, or completeness of any such publicly available information.

Pursuant to the merger agreement dated November 12, 2015, First Midwest has agreed to exchange $71.0 million in cash and common stock for all of the outstanding shares of common stock and options of NI Bancshares.  The consideration will consist of approximately $15.3 million in cash and $55.7 million in common stock of First

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Midwest.  The number of shares of First Midwest common stock to be issued was based upon the average of the prices of the First Midwest common stock on the fifteen consecutive NASDAQ Stock Market trading days on a volume weighted average basis ending on and including the trading day immediately preceding the date of the merger agreement, which equaled $18.2569 per share.  The value and the composition of the total merger consideration may be adjusted pursuant to the terms of the merger agreement.  In the event that the 15-day average volume weighted price of First Midwest common stock has declined by more than 20% from the date of the agreement and First Midwest common stock underperforms the KBW Regional Banking index by more than 15% during the same time period, NI Bancshares can elect to terminate the merger agreement.  If NI Bancshares would exercise this right, First Midwest would have the option to cure or “top-up” the total consideration by increasing either the stock consideration or the cash consideration, or both.

NI Bancshares Discounted Cash Flow Analysis:  Using discounted cash flow analysis, Sheshunoff estimated the present value of the future after-tax cash flow streams that NI Bancshares could produce on a stand-alone basis through September 30, 2020 under various circumstances, assuming that it performed in accordance with the projections provided by NI Bancshares’ management.

Sheshunoff estimated the terminal value for NI Bancshares at the end of September 30, 2020 by (1) multiplying the final period projected earnings by one plus the assumed annual long-term growth rate of the earnings of NI Bancshares of 4.0% (or 1.04) and (2) dividing this product by the difference between the required rates of return shown below and the assumed annual long-term growth rate of earnings of 4.0% in (1) above.  Sheshunoff discounted the annual cash flow streams (defined as all earnings in excess of that which is required to maintain a tangible common equity to tangible asset ratio of 8.0%) and the terminal values using discount rates ranging from 12.0% to 14.0%.  The discount rate range was chosen to reflect different assumptions regarding the required rates of return of NI Bancshares and the inherent risk surrounding the underlying projections.  This discounted cash flow analysis indicated a range of values per share of $49.48 to $62.88 as shown in the table below compared to the estimated merger consideration of $65.86 per share.

Analysis of Selected Transactions:  Sheshunoff performed an analysis of premiums paid in selected recently announced acquisitions of banking organizations with comparable characteristics to NI Bancshares.  Two sets of transactions were selected to ensure a thorough analysis.

The first set of comparable transactions consisted of a group of selected transactions for banks and thrifts in the United States for which pricing data was available, with the following characteristics: total assets between $100 million and $2 billion that were announced since January 1, 2014, reporting a return on average assets between 0.0% and 0.50%, a non-performing assets to total assets ratio less than 3.5%, and a loan to deposit ratio less than 100%.  These comparable transactions consisted of 47 mergers and acquisitions of banks and thrifts with total assets ranging between $101.8 million and $1.74 billion that were announced between January 14, 2014 and October 23, 2015.  The analysis yielded multiples of the purchase prices in these transactions as summarized below:

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The median pricing multiples in the comparable transactions approximated or were lower than those in connection with the merger, except the median price to assets and price to deposits basis, which were slightly higher due primarily to NI Bancshares exhibiting lower capital levels compared to the median level of the target institutions of the comparable transactions.  The median premium to deposits in the comparable transactions was slightly higher than in the merger.

The second set of comparable transactions consisted of a group of selected transactions for banks and thrifts headquartered in the Midwest Region of the United States for which pricing data was available, with the following characteristics: total assets between $100 million and $2 billion that were announced since January 1, 2014, a return on average assets between 0.0% and 0.50%, non-performing assets to total assets ratio less than 3.5%, and a loan to deposit ratio less than 100%.  These comparable transactions consisted of seven mergers and acquisitions of banks and thrifts with total assets ranging between $101.8 million and $442.4 million that were announced between March 10, 2014 and July 28, 2015.  The analysis yielded multiples of the purchase prices in these transactions as summarized below:

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The median pricing multiples in the comparable transactions were lower than those in the merger, except the median price to assets and median price to deposits, which were slightly higher due primarily to NI Bancshares exhibiting lower capital levels compared to the median level of the target institutions of the comparable transactions.

Contribution Analysis:  Sheshunoff reviewed the relative contributions of NI Bancshares and First Midwest to the combined company based on regulatory data which has not been audited as of September 30, 2015 for NI Bancshares and First Midwest.  Sheshunoff compared the pro forma ownership interests (which excludes the cash component of the merger consideration) of NI Bancshares and First Midwest of 3.8% and 96.2%, respectively, to: (1) total assets of 6.4% and 93.6%, respectively; (2) total loans of 5.7% and 94.3%, respectively; (3) total deposits of 6.8% and 93.2%, respectively; (4) net-interest income of 5.8% and 94.2%, respectively; (5) non-interest income of 7.5% and 92.5%, respectively; (6) non-interest expenses of 8.2% and 91.8%, respectively; (7) September 30, 2015 LTM earnings of 1.7% and 98.3%, respectively; and (8) total tangible equity of 6.3% and 93.7%, respectively.  The contribution analysis shows that the ownership of NI Bancshares stockholders in the combined company is less than the contribution of the components listed (with the exception of earnings) due largely to the considerable lower level of earnings and the amount of cash consideration in the merger.  The contributions are shown in the following table.