A project by North American and NASA to construct a prototype to reach a top speed of Mach 6 or 4,520 mph (7,274 km/h) at an altitude of almost 100,000 feet in 1961.
Video: https://www.youtube.com/watch?v=rAvyLfC6FSM
IMO, it was our first space plane, though sub orbital, it often exceeded an altitude of 50 miles. X-15 pilots were awarded Astronaut Wings when they exceeded 50 miles.Nov 19, 2002.