The Martin Marietta X-24B was an experimental US aircraft developed from a joint USAF-NASA program named PILOT(1963–1975). It was designed and built to test lifting body concepts, experimenting with the concept of un-powered reentry and landing, later used by the Space Shuttle. The X-24 was drop launched from a modified B-52 Stratofortress at high altitudes before igniting its rocket engine; after expending its rocket fuel, the pilot would glide the X-24 to an un-powered landing.