X-planes are United States experimental aircraft designed to answer fundamental questions about the behavior of aircraft close to, at, or beyond the speed of sound, and as prototypes for advanced aerospace vehicles. Most of these planes have been flown from Muroc Air Field, later named Edwards Air Force Base.
The "X" designation, originally "XS" for eXperimental Supersonic, applied to a family of experimental aircraft not intended for production beyond a limited number built solely for flight research. The D-558-1 and -2 did not bear the "X" label but were clearly intended for the same purpose. This was also true of the non-X-designated lifting bodies, whereas the XB-70 was originally intended to be a production bomber, and the XF-92A was expected to be a production fighter. Neither of the two latter aircraft actually went into production, and both models became significant research aircraft, hence their inclusion here.
For more information on specific X-planes, see: