Also known as the X-1. This rocket plane was the first aircraft to break the sound barrier, and the first in a line of X- aircraft leading to the space shuttle. Due to the press of war work, Bell Aircraft was the only company that would take on the project in 1944. German swept-wing research being unknown, the configuration developed was a thin, straight-winged aircraft with a bullet-shaped fuselage. The XS-1 began flight test in 1946 and Chuck Yeager flew it through the sound barrier on October 14, 1947.

X-2 was an AAF/ Bell project that flew three supersonic flight research aircraft, powered by liquid rockets. Originally designated XS-2. The X-2 was the first swept-wing X rocketplane. It exceeded Mach 3, but in the course of doing so uncovered the supersonic aircraft problem of inertial coupling. On its last flight the aircraft crashed and the pilot was killed.

The D-558-I "Skystreaks" were among the early transonic research airplanes like the X-1, X-4, X-5, and XF-92A. Three of the single-seat, straight-wing aircraft flew in a joint program involving the National Advisory Committee for Aeronautics (NACA), with its flight research done at the NACA�s Muroc Flight Test Unit in Calif., redesignated in 1949 the High-Speed Flight Research Station; the Navy-Marine Corps; and the Douglas Aircraft Co. from 1947 to 1953. In the process, the Skystreaks managed to set a couple of world speed records.

A crashed X-15 no. 2 was rebuilt to attain even higher speeds. The body frame was stretched, two drop tanks were added, increasing fuel load by 75%. An ablative heat shield was applied for the high speeds that could be attained. The X-15A-2 reached the highest speeds and altitudes of any manned spaceplane until the space shuttle entered service.