XB-70 Valkyrie

The idea behind the XB-70 Valkyrie was to lay the groundwork for development of a strategic bomber, but in the end, this X-Plane was used primarily as a test bed for potential supersonic transport (SST) aircraft for civilian travel. North American Aviation built two of the XB-70s, which together between 1964 and 1969 made 129 flights.

The design was intended for flight at Mach 3, but it proved less than ideal at that speed, and the two aircraft together logged less than 2 hours of Mach 3 flight time. Along with insights into handling at supersonic speeds, the XB-70 provided a great deal of information about sonic booms and other noise factors that would be important for commercial flights by supersonic aircraft.