The Falls Church, Virginia based company announced the appointment of Mattis, 68, on Wednesday, amid ongoing controversy over the cozy relationship between Pentagon leaders and the defense industry.

"Jim is a thoughtful, deliberate and principled leader with a proven track record of selfless service to our nation," said company chairman and chief executive Phebe Novakovic.

"We are honored to have him on our board."

A veteran soldier with a 43-year career in the Marines and a sparing, intellectual lifestyle that earned him the label "warrior monk," Mattis led the Pentagon for the first two years of the Trump administration, resisting open conflict with the White House despite major disagreements over defense and security policy.

He stepped down in December 2018 with a broadside clearly directed at Trump.

"My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues," he said in a letter of resignation.

After quitting the Pentagon, he has remained out of the public eye, becoming a fellow at the Hoover Institution think-tank at Stanford University.

He was previously on the General Dynamics board during his short-lived retirement from the military from August 2013 to January 2017.

Both of the people Trump named to replace him have courted controversy over their close ties to the defense industry.

Trump first chose Mattis's Pentagon deputy Patrick Shanahan, but he ran into controversy as a career executive at aircraft maker Boeing, which hold billions of dollars worth of contracts with the Defense Department.

Shanahan eventually withdrew his name for personal reasons, and Trump nominated Mark Esper, a former soldier who spent years at defense contractor Raytheon. Esper was confirmed in July.