President Trump began his presidency surrounding himself with military generals in key Cabinet posts. But his pick for acting defense secretary, Patrick Shanahan, 46, is a sharp departure from that mold.

Mr. Trump announced on Twitter Sunday that Shanahan, his deputy defense secretary, would replace James Mattis as of Jan. 1. "Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing," Trump said. "He will be great!"

Shanahan came to the defense department in 2017 after a three-decades-long career working at aerospace giant Boeing Co. He most recently worked as senior vice president of supply chain and operations, and previously worked as senior vice president of commercial airplane programs, according to DOD.

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His background is in mechanical engineering, with two advanced degrees from the Massachusetts Institute of Technology. During his work in the administration, he has advocated for Mr. Trump's effort to create a sixth military branch, known as the "Space Force."

Mattis, in contrast, is a retired Marine general. When Mr. Trump became president, he touted Mattis, as well as General John Kelly, as an important military presence in his inner circle. Kelly, his original homeland security secretary who later became Mr. Trump's chief of staff, is also leaving at the end of the year.

As acting defense secretary, Shanahan will not need to be confirmed by the Senate. Shanahan was confirmed as deputy defense secretary in 2017, though former Sen. John McCain at the time threatened to block his appointment over his lack of Defense Department experience.

Other military generals that were once part of Mr. Trump's inner circle as president were Michael Flynn, his original national security adviser, who was later fired and recently pleaded guilty to lying to the FBI, and H.R. McMaster, his second national security adviser.