President Trump said Defense Secretary James Mattis will leave his job sooner than had been planned last week, and he is appointing Deputy Secretary Patrick Shanahan as acting secretary of defense. In a tweet Sunday, Mr. Trump said Shanahan would step into the new role on Jan. 1.

I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019. Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great! — Donald J. Trump (@realDonaldTrump) December 23, 2018

CBS News correspondent David Martin first reported the news, citing a senior administration official, after discussing the topic on "Face the Nation" Sunday.

Mattis had planned to work through the February 28 date he cited in his resignation letter. Mattis abruptly announced he would resign after Mr. Trump stunned advisers with this week's decision to pull U.S. troops out of Syria.The retired Marine general wrote in his resignation letter, "You have the right to have a secretary of defense whose views are better aligned with yours."

Shanahan, a longtime Boeing executive who has been Mattis' deputy since the summer of 2017, will accept the appointment as acting secretary, according to his spokesperson Lt. Col. Joseph Buccino.

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"Deputy Secretary will continue to serve as directed by the president, and the Department of Defense will remain focused on the defense of the nation," Buccino said on Sunday.

The Puget Sound Business Journal called Shanahan a Boeing "fix-it" man in a March 2016 report. He oversaw the company's global supply chain strategies and use of advanced manufacturing technologies. Shanahan was central to getting the 787 Dreamliner on track after production problems in the program's early years, the report said.