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SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

SCHEDULE 13D

[Rule 13d-101]

INFORMATION TO BE INCLUDED IN STATEMENTS FILED

PURSUANT TO § 240.13d-1(a) AND AMENDMENTS

THERETO FILED PURSUANT TO § 240.13d-2(a)

Amendment No. 18

Navistar International Corporation

(Name of Issuer)

Common Stock, par value $0.10 per share

(Title of Class of Securities)

63934E108

(CUSIP Number)

Janet Yeung

MHR Fund Management LLC

1345 Avenue of the Americas, 42^nd Floor

New York, New York 10105

(212) 262-0005

(Name, Address and Telephone Number of Person Authorized to Receive Notices of Communication)

September 21, 2016

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box. ¨

Note: Schedules filed in paper format shall include a signed original and five copies of the Schedule, including all exhibits. See §240.13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (the “Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

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Explanatory Note

This statement on Schedule 13D (this “Statement”) amends and supplements, as Amendment No. 18, the Schedule 13D filed on June 15, 2012 (the “Initial Schedule 13D”), which was amended on June 25, 2012 by Amendment No. 1 to the Initial Schedule 13D (“Amendment No. 1”), on July 10, 2012 by Amendment No. 2 to the Initial Schedule 13D (“Amendment No. 2”), on October 9, 2012 by Amendment No. 3 to the Initial Schedule 13D (“Amendment No. 3”), on October 26, 2012 by Amendment No. 4 to the Initial Schedule 13D (“Amendment No. 4”), on December 12, 2012 by Amendment No. 5 to the Initial Schedule 13D (“Amendment No. 5”), on July 15, 2013 by Amendment No. 6 to the Initial Schedule 13D (“Amendment No. 6”), on July 17, 2013 by Amendment No. 7 to the Initial Schedule 13D (“Amendment No. 7”), on March 11, 2014 by Amendment No. 8 to the Initial Schedule 13D (“Amendment No. 8”), on April 11, 2014 by Amendment No. 9 to the Initial Schedule 13D (“Amendment No. 9”), on June 24, 2015 by Amendment No. 10 to the Initial Schedule 13D (“Amendment No. 10”), on December 18, 2014 by Amendment No. 11 to the Initial Schedule 13D (“Amendment No. 11”), on January 30, 2015 by Amendment No. 12 to the Initial Schedule 13D (“Amendment No. 12”), on June 11, 2015 by Amendment No. 13 to the Initial Schedule 13D (“Amendment No. 13”), on July 27, 2015 by Amendment No. 14 to the Initial Schedule 13D (“Amendment No. 14”), on September 4, 2015 by Amendment No. 15 to the Schedule 13D (“Amendment No. 15”), on December 18, 2015 by Amendment No. 16 to the Initial Schedule 13D (“Amendment No. 16”) and on September 6, 2016 by Amendment No. 17 to the Initial Schedule 13D (“Amendment No. 17” and together with Amendment No. 1 through Amendment No. 16 and the Initial Schedule 13D, the “Schedule 13D”), and relates to shares of common stock, par value $0.10 per share (the “Common Stock”), of Navistar International Corporation (the “Issuer”). Except as otherwise provided, capitalized terms used in this Statement but not defined herein shall have the respective meanings given to such terms in Amendment No. 17.

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Item 4. Purpose of Transaction

Item 4 is hereby amended by adding the following:

The information set forth in Item 6 below is incorporated into this Item 4 by reference.

Item 6. Contracts, Arrangements, Understandings or Relationships With Respect to the Securities of the Issuer.

Item 6 is hereby amended by adding the following:

On September 21, 2016, at the Issuer’s request, Master Account, Capital Partners (100) and Institutional Partners III (collectively, the “MHR Funds”) agreed to the Issuer’s request to waive their rights under that certain Registration Rights Agreement, dated October 5, 2012, by and among the Issuer and the MHR Funds (the “Registration Agreement”), to include securities held by them as selling securityholders in the Issuer’s registration statement on Form S-3, filed on September 22, 2016 (the “Registration Statement”), or in any subsequent offering by the Issuer with respect to the Issuer’s securities registered under such Registration Statement. At the same time, the MHR Funds and the Issuer entered into Amendment No. 1 to the Registration Agreement (“Amendment No. 1”), pursuant to which the aggregate number of demand registrations that the Issuer is obliged to effect on behalf of the MHR Funds increased from three to four.

The foregoing description of Amendment No. 1 does not purport to be complete and is qualified in its entirety by reference to the full text of Amendment No. 1, a copy of which is filed as Exhibit 1 to this Statement and is incorporated by reference herein.

Item 7. Material to be Filed as Exhibits

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SIGNATURES

After reasonable inquiry and to the best of our knowledge and belief, the undersigned certify that the information set forth in this Statement is true, complete and correct.

Dated: September 22, 2016

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