Air Transport Services Group Inc. (ATSG): Red Mountain Capital Agrees to Sell 3.8 Million Shares Back to Company

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE
13D/A

Under the Securities Exchange Act of 1934

(Amendment No. 12)^*

AIR TRANSPORT SERVICES GROUP, INC.

(Name of Issuer)

Common
Stock, par value $0.01 per share

(Title of Class of Securities)

00922R105

(CUSIP Number)

Red Mountain Capital Partners LLC

Attn: Willem Mesdag

10100 Santa Monica Boulevard, Suite 925

Los Angeles, California 90067

Telephone (310) 432-0200

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

June 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 Rule 13d-1(e), 13d-1(f) or 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 Rule 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 (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).

* See Instructions

* See Instructions

* See Instructions

* See Instructions

* See Instructions

This Amendment No. 12 amends and supplements the Schedule 13D filed with the Securities
and Exchange Commission (the SEC) on November 20, 2006, as amended by Amendment No. 1 thereto, filed with the SEC on September 24, 2007, Amendment No. 2 thereto, filed with the SEC on February 7, 2008, Amendment
No. 3 thereto, filed with the SEC on December 3, 2008, Amendment No. 4 thereto, filed with the SEC on January 6, 2009, Amendment No. 5 thereto, filed with the SEC on February 3, 2009, Amendment No. 6 thereto, filed
with the SEC on May 26, 2009, Amendment No. 7 thereto, filed with the SEC on June 1, 2009, Amendment No. 8 thereto, filed with the SEC on August 29, 2011, Amendment No. 9 thereto, filed with the SEC on June 18,
2012, Amendment No. 10 thereto, filed with the SEC on August 14, 2012, and Amendment No. 11 thereto, filed with the SEC on March 14, 2016 (together, this Schedule 13D), by (i) Red Mountain Capital Partners LLC, a
Delaware limited liability company (RMCP LLC), (ii) Red Mountain Partners, L.P., a Delaware limited partnership (RMP), (iii) RMCP GP LLC, a Delaware limited liability company (RMCP GP), (iv) Red
Mountain Capital Management, Inc., a Delaware corporation (RMCM), and (v) Willem Mesdag, a natural person and citizen of the United States of America, with respect to the common stock, par value $0.01 per share (the Common
Stock), of Air Transport Services Group, Inc., a Delaware corporation (ATSG), formerly known as ABX Holdings, Inc. and ABX Air, Inc.

RMCP LLC, RMP and RMCP GP are sometimes collectively referred to herein as Red Mountain. Red Mountain, RMCM and Mr. Mesdag
are sometimes collectively referred to herein as the Reporting Persons. The filing of any amendment to this Schedule 13D (including the filing of this Amendment No. 12) shall not be construed to be an admission by the Reporting
Persons that a material change has occurred in the facts set forth in this Schedule 13D or that such amendment is required under Rule 13d-2 of the Securities Exchange Act of 1934, as amended.

Items 5(a) and 5(b) of this Schedule 13D are hereby amended and
restated as follows:

Because each of RMCP GP, RMCP LLC, RMCM and Mr. Mesdag may be deemed to control RMP, each of RMCP GP, RMCP LLC, RMCM and Mr. Mesdag
may be deemed to beneficially own, and to have the power to vote or direct the vote, or dispose or direct the disposition of, all of the Common Stock beneficially owned by RMP. As a result, RMCP GP, RMCP LLC, RMCM and Mr. Mesdag may be deemed
to beneficially own, in the aggregate, 11,152,425 shares of Common Stock, representing 17.5% of the outstanding Common Stock.⁽²⁾

Other than shares of Common Stock beneficially owned by RMP, and other than, in the case of Mr. Teets, shares of Common Stock underlying
restricted stock units granted to Mr. Teets in connection with his service on the board of directors of ATSG, none of the Reporting Persons or Mr. Teets may be deemed to beneficially own any shares of Common Stock.

Each of RMCP LLC, RMP and RMCP GP affirms membership in a group with each other but
disclaims membership in a group with RMCM or Mr. Mesdag. Each of RMCM and Mr. Mesdag disclaims membership in a group with any person.

The filing of this Schedule 13D shall not be construed as an admission that any Reporting Person is the beneficial owner of any of the shares
of Common Stock that such Reporting Person may be deemed to beneficially own. Without limiting the foregoing sentence, each of RMCM and Mr. Mesdag disclaims beneficial ownership of all shares of Common Stock reported in this Schedule 13D. In
addition, the filing of this Schedule 13D shall not be construed as an admission that any partner, member, director, officer or affiliate of any Reporting Person is the beneficial owner of any of the shares of Common Stock that such partner, member,
director, officer or affiliate may be deemed to beneficially own. Without limiting the foregoing sentence, Mr. Teets disclaims beneficial ownership of all shares of Common Stock reported in this Schedule 13D.

Item 6 of this Schedule 13D is hereby amended to include the following information:

On June 21, 2016, RMP entered into a securities purchase agreement with ATSG (the Securities Purchase Agreement), pursuant to
which RMP agreed to sell, and ATSG agreed to repurchase, 3,825,554 shares of Common Stock for an aggregate purchase price of $50,000,000. The consummation of the transactions contemplated by the Securities Purchase Agreement is expected to occur on
July 5, 2016, except to the extent otherwise provided in the Securities Purchase Agreement.

The foregoing references to and
description of the Securities Purchase Agreement do not purport to be complete and are subject, and are qualified in their entirety by reference, to the full text of the Securities Purchase Agreement, which is incorporated by reference in this
Item 6.

Item 7 of this Schedule 13D is hereby amended to include the
following information:

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true,
complete and correct.

Dated: June 23, 2016

EXHIBIT INDEX