On July 2, 1996, NASA selected Lockheed Martin to design, build and fly the X-33 test vehicle between March and December 1999.

The X-33 design was based on a lifting body shape with two revolutionary "linear aerospike" rocket engines and a rugged metallic thermal protection system. The vehicle also was to have had lightweight components and fuel tanks built to conform to the vehicle's outer shape. Time between X-33 flights was planned to normally be seven days, but the program hoped to demonstrate a two-day turnaround between flights during the flight-test phase of the program.

The X-33 was to have been an unpiloted vehicle that took off vertically like a rocket and landed horizontally like an airplane. It was planned to reach altitudes of up to 50 miles and high hypersonic speeds. The X-33 Program was managed by the Marshall Space flight Center and was planned to have been launched at a special launch site on Edwards Air Force Base. Technical problems with the composite liquid hydrogen tank resulted in the program being cancelled in February 2001.