The X-planes are a series of experimental United States airplanes and helicopters (and some rockets) used for the testing and evaluation of new technologies and aerodynamic concepts. Some members of the series have been well publicised, while others, such as the X-16 project, have been developed in secrecy.
The first of the X-planes, the Bell X-1, became well known as, in 1947, it was the first aircraft to break the sound barrier in level flight. Later X-planes yielded important research results 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 7 through 12 were actually missiles, and some other vehicles were unpiloted. Most X-planes are not expected to go into full-scale production, however an exception to the rule was the Lockheed Martin X-35, which competed against the Boeing X-32 in a fly-off as part of the Joint Strike Fighter Program, and is expected to enter production as the F-35.
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