[1] The program ran from October 24, 1957, to December 10, 1963, cost US$660 million ($6.57 billion in current dollars[2]), and was cancelled just after spacecraft construction had begun.

It could travel to distant targets at the speed of an intercontinental ballistic missile, was designed to glide to Earth like an aircraft under the control of a pilot, and could land at an airfield.

The concept underlying the X-20 was developed in Germany during World War II by Eugen Sänger and Irene Bredt as part of the 1941 Silbervogel proposal.

This was a design for a rocket-powered bomber able to attack New York City from bases in Germany and then fly on for landing somewhere in the Pacific Ocean held by the Empire of Japan.

[7] Following the war, many German scientists were taken to the United States by the Office of Strategic Services's Operation Paperclip, bringing with them detailed knowledge of the Silbervogel project.

[8] Among them, Walter Dornberger and Krafft Ehricke moved to Bell Aircraft, where, in 1952, they proposed what was essentially a vertical launch version of Silbervogel known as the "Bomber Missile", or "BoMi".

[9][10] These studies all proposed various rocket-powered vehicles that could travel vast distances by gliding after being boosted to high speed and altitude by a rocket stage.

A Martin Marietta Transtage upper stage attached to the aft end of the craft would allow orbital maneuvers and a launch abort capability before being jettisoned before descent into the atmosphere.

While falling through the atmosphere an opaque heat shield made from a refractory metal would protect the window at the front of the craft.

This would be a unique ability for a spacecraft, as the laws of celestial mechanics ordinarily mean a change of plane requires an enormous expenditure of energy.

[26] By the end of 1962, Dyna-Soar had been designated X-20, the booster (to be used in the Dyna Soar I drop-tests) successfully fired, and the USAF had held an unveiling ceremony for the X-20 in Las Vegas.

[27][28] The Minneapolis-Honeywell Regulator Company (later the Honeywell Corporation) completed flight tests on an inertial guidance sub-system for the X-20 project at Eglin Air Force Base, Florida, utilizing an NF-101B Voodoo by August 1963.

The original USAF proposal suggested LOX/JP-4, fluorine-ammonia, fluorine-hydrazine, or RMI (X-15) engines, but Boeing, the principal contractor, favored an Atlas-Centaur combination.

On January 19, 1963, the Secretary of Defense, Robert McNamara, directed the U.S. Air Force to undertake a study to determine whether Gemini or Dyna-Soar was the more feasible approach to a space-based weapon system.

In the middle of March 1963, after receiving the study, Secretary McNamara "stated that the Air Force had been placing too much emphasis on controlled re-entry when it did not have any real objectives for orbital flight".