Douglas X-3 Stiletto. The twin-turbojet X-3, the only one built, was designed to test sustained flight at twice the speed of sound. It also explored the use of very short wings and titanium airframe construction. Engine development difficulties forced the use of lower powered engines than originally planned, prohibiting the X-3 from achieving its Mach 2 design potential. Even so, data gained from the X-3 program greatly benefited the F-104, X-15, SR-71 and other high performance aircraft.