The Boeing X-20 Dyna-Soar ("Dynamic Soarer") was a United States Air Force (USAF) program to develop a spaceplane that could be used for a variety of military missions, including aerial reconnaissance, bombing, space rescue, satellite maintenance, and as a space interceptor to sabotage enemy satellites. The program ran from October 24, 1957, to December 10, 1963, cost US$660 million ($5.84 billion in current dollars), and was cancelled just after spacecraft construction had begun.

Other spacecraft under development at the time, such as Mercury or Vostok, were space capsules with ballistic re-entry profiles that ended in a landing under a parachute. Dyna-Soar was more like an aircraft. It could travel to distant targets at the speed of an intercontinental ballistic missile, was designed to glide to Earth like an aircraft under control of a pilot, and could land at an airfield. Dyna-Soar could also reach Earth orbit, like conventional, manned space capsules.These characteristics made Dyna-Soar a far more advanced concept than other human spaceflight missions of the period. Research into a spaceplane was realized much later in other reusable spacecraft such as the 19812011 Space Shuttle and the more recent Boeing X-40 and X-37B spacecraft.

The United States Air Force (USAF) is the air service branch of the United States Armed Forces, and is one of the eight U.S. uniformed services. Initially formed as a part of the United States Army on 1 August 1907, the USAF was established as a separate branch of the U.S. Armed Forces on 18 September 1947 with the passing of the National Security Act of 1947. It is the second youngest branch of the U.S. Armed Forces and the fourth in order of precedence. The U.S. Air Force articulates its core missions as air supremacy, global integrated intelligence, surveillance and reconnaissance, rapid global mobility, global strike, and command and control.

The U.S. Air Force is a military service branch organized within the Department of the Air Force, one of the three military departments of the Department of Defense. The Air Force through the Department of the Air Force is headed by the civilian Secretary of the Air Force, who reports to the Secretary of Defense and is appointed by the President with Senate confirmation. The highest-ranking military officer in the Air Force is the Chief of Staff of the Air Force, who exercises supervision over Air Force units and serves as one of the Joint Chiefs of Staff. As directed by the Secretary of Defense and Secretary of the Air Force, certain Air Force components are assigned to unified combatant commands. Combatant commanders are delegated operational authority of the forces assigned to them, while the Secretary of the Air Force and the Chief of Staff of the Air Force retain administrative authority over their members.

Along with conducting independent air operations, the U.S. Air Force provides air support for land and naval forces and aids in the recovery of troops in the field. As of 2017, the service operates more than 5,369 military aircraft and 406 ICBMs. It has a $156.3 billion budget and is the second largest service branch, with 329,614 active duty airmen, 172,857 civilian personnel, 69,056 reserve airmen, and 107,414 Air National Guard airmen.