The U.S. government is replacing a whole host of military equipment worn out after the wars in Afghanistan and Iraq as well as Cold War era hardware that is becoming obsolete. In 2015, the Pentagon's top-100 contractors had obligated contracts worth $175 billion, according to government figures. Lockheed Martin, who are currently developing the F-35 Joint Strike Fighter, were the largest government contractor by far with $36.2 billion contracted. Of that, $29.4 billion was contracted for defense projects. The F-35 is the most expensive U.S. weapons system ever, costing some $400 billion and due to cost $1 trillion more over the course of its life cycle.
This chart shows U.S. Department of Defense expenditure with contractors in 2015.