An F-106A Delta Dart aircraft passes over the Mojave Desert while en route to Davis-Monthan Air Force Base, Ariz., where it will be used in the QF-106 drone program. The aircraft, which was the second-to-last F-106 in active service, had been used as a safety chase aircraft in the B-1B aircraft production acceptance flight test program

The X-planes are a series of experimental United States aircraft and rockets, used to test and evaluate new technologies and aerodynamic concepts. They have an X designator, which indicates the research mission within the US system of aircraft designations. The first, the Bell X-1, became well known in 1947 after it became the first aircraft to break the sound barrier in level flight. Most of the X-planes have been operated by the National Advisory Committee for Aeronautics (NACA) or, later, the National Aeronautics and Space Administration (NASA), often in conjunction with the United States Air Force. The majority of X-plane testing has occurred at Edwards Air Force Base. Some of the X-planes have been well publicized, while others have been developed in secrecy. Most X-planes are not expected to go into full-scale production.