XV-15 Tilt Rotor (NASA-703) in flight at Ames Research Center Note: Used in publication in Flight Research at Ames; 57 Years of Development and Validation of Aeronautical Technology NASA SP-1998-3300 fig 122 ARC-1985-AC85-0186-2

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.