Since 1946, NACA (NASA's predecessor) and, since 1958, NASA have conducted the X-Plane Program. The program was originally intended to create a family of experimental aircraft not intended for production beyond the limited number of each design built solely for flight research. The first X-Plane, the Bell X-1, was the first rocket-powered airplane to break the sound barrier on October 14, 1947. X-Planes have set numerous milestones since then, both manned and unmanned.