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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.
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, such as the X-16, have been developed in secrecy. 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. Later X-planes supported important research in a multitude of aerodynamic and technical fields, but only the North American X-15 rocket plane of the early 1960s achieved comparable fame to that of the X-1. X-planes 8, 9, 11, 12, and 17 were actually missiles used to test new types of engines, and some other vehicles were un-manned (some were remotely flown, some were partially or fully autonomous).
Most X-planes are not expected to go into full-scale production; one exception was the Lockheed Martin X-35, which competed against the Boeing X-32 during the Joint Strike Fighter Program, and has entered production as the F-35.
Not all US experimental aircraft have been designated as X-planes; some received US Navy designations before 1962, while others have been known only by manufacturers' designations, non-'X'-series designations, or classified codenames.