WASHINGTON — President Donald Trump on Thursday said he will nominate Patrick Shanahan to be his second secretary of defense. Shanahan, a former Boeing executive, has been leading the Pentagon as acting secretary since Jan. 1, a highly unusual arrangement for arguably the most sensitive Cabinet position.

"Acting Secretary Shanahan has proven over the last several months that he is beyond qualified to lead the Department of Defense, and he will continue to do an excellent job," White House press secretary Sarah Sanders said in a statement.

Moments later, Shanahan issued a statement saying he is honored by the announcement and, if confirmed, "I will continue the aggressive implementation of our National Defense Strategy. I remain committed to modernizing the force so our remarkable Soldiers, Sailors, Airmen, and Marines have everything they need to keep our military lethal and our country safe."

Shanahan wields none of the star power of Trump's first defense secretary, Jim Mattis. And that may be just the thing for a commander in chief who seemed to resent Mattis for his reputation in Washington as a superior strategist and a moderating influence on an impulsive president.

Two months before Mattis resigned, citing policy differences, Trump publicly questioned whether he was "sort of a Democrat." After the former Marine general quit, Trump spoke more harshly, calling Mattis a failure and insisted he had fired him, even though Mattis had resigned first.

Shanahan, 56, has a lifetime of experience in the defense industry but little in government. In more than three months as the acting secretary, he has focused on implementing the national defense strategy that was developed during Mattis' tenure and emphasizes a shift from the resources and tactics required to fight small wars against extremist groups to what Shanahan calls "great power" competition with China and Russia.

The Shanahan nomination is not known to face any organized opposition in Congress, although some members have been lukewarm on him. Sen. James Inhofe, the Oklahoma Republican who is chairman of the Senate Armed Services Committee, has questioned why Trump was taking so long to seek the Senate's consent but has indicated he would support Shanahan.

This is only the third time in history that the Pentagon has been led by an acting chief. The last was William H. Taft, who served for two months in 1989 after President George H.W. Bush's first choice to be defense secretary, John Tower, became mired in controversy and ultimately failed to be confirmed by the Senate. Dick Cheney, the future vice president under President George W. Bush, then was nominated and confirmed.

Presidents typically take pains to ensure the Pentagon is being run by a Senate-confirmed official, given the grave responsibilities that include sending young Americans into battle, ensuring the military is ready for extreme emergencies like nuclear war and managing overseas alliances that are central to U.S. security.

Shanahan, who grew up in Seattle, earned a Bachelor of Science degree in mechanical engineering from the University of Washington and two advanced degrees from the Massachusetts Institute of Technology. He joined Boeing in 1986, rose through its ranks and is credited with rescuing a troubled Dreamliner 787 program. He also led the company's missile defense and military helicopter programs.

His prospects for being nominated seemed to take a hit when the Defense Department's inspector general opened an investigation March 15 in response to allegations that Shanahan had shown favoritism toward Boeing during his time as deputy defense secretary, while disparaging Boeing competitors.

Late last month the inspector general's office said its probe found no basis for the allegations and concluded that Shanahan had lived up to his ethics obligations. The IG interviewed Shanahan as well as 33 witnesses under oath, including Mattis and members of the Joint Chiefs of Staff.

"We did not substantiate any of the allegations. We determined that Mr. Shanahan fully complied with his ethics agreements and his ethical obligations regarding Boeing and its competitors," the report said.

Trump has seemed attracted to Shanahan partially for his work on one of the president's pet projects — creating a Space Force. Trump also has publicly lauded Boeing, builder of many of the military's most prominent aircraft, including the Apache and Chinook helicopters, the C-17 cargo plane and the B-52 bomber, as well as the iconic presidential aircraft, Air Force One.

Although a few members of the Senate have rhetorically roughed up Shanahan, he has not generated broad opposition during his months of auditioning for the nomination. Sen. Lindsey Graham, a South Carolina Republican, butted heads with Shanahan over the administration's Syria policy, but that confrontation quickly faded after the White House partially reversed course by agreeing to keep a few hundred troops in Syria rather than withdrawing all 2,000.

Shanahan was the deputy secretary of defense during Mattis' two-year tenure. No one thought of him then as a potential No. 1 since he had never previously served in government and carried little political sway in Washington or in foreign capitals. Aides say that during his 17 months as deputy, Shanahan was deeply engaged in the full range of policy issues and briefed on military operations. He shares Mattis's conviction that the Pentagon needs to shift its focus from fighting insurgent wars to preparing for and deterring armed conflict with big powers like China.

“China, China, China,” was his message to senior department officials the day he took over from Mattis, aides said.