Shanahan had a long career at Boeing, rising to a vice president of the major US armsmaker before taking up with the Trump Administration as a Deputy Defense Secretary, and later Acting Defense Secretary. Which is not to say that the politically powerful arms industry is losing the post, as it has been announced that Mark Esper, himself a former vice president for Raytheon, will be taking over the post. Esper was previously Secretary of the Army.
https://news.antiwar.com/2019/06/18/boeing-out-raytheon-in-shanahan-quits-as-acting-secdef/
