Lawmakers have previously expressed concern about undue influence on the Pentagon from defense companies.

Ties between Boeing and the Trump administration run deep, with Trump using the company's products and sites as a backdrop for major announcements.

Boeing, the world's largest planemaker, is facing one of the biggest crises in its 103-year history following the disasters on Lion Air in Indonesia last year and another on Ethiopian Airlines in March, which together killed all 346 on board.

The Pentagon inspector general started the investigation of Shanahan in March for allegedly promoting Boeing in Pentagon meetings and disparaging competitors. But a report published on April 25 said none of the allegations were substantiated.

Defense secretaries have traditionally come from a political or policy background, serving as a counterweight to the military brass in decision making. Mattis, a retired Marine general, was one of the few former military leaders to become secretary.

Mattis implicitly criticized Trump in his resignation letter for failing to value allies who fight alongside the United States, including in places like Syria.

Shanahan is unlikely to be an effective counterweight to Trump's often impulsive decision making. He toured the US border with Mexico in February in what was seen as a show of support for Trump's planned border wall.