The Northrop XB-35 and YB-35 were experimental heavy bomber aircraft that used a new radical and potentially very efficient flying wing design, in which the tail section and fuselage are eliminated and all payload is carried in a thick wing. Only prototype and pre-production aircraft were built, although interest remained strong enough to warrant further development of the aircraft as a jet bomber, under the designation YB-49.
The North American X-15 was a rocket-powered aircraft operated by the United States Air Force and the National Aeronautics and Space Administration as part of the X-plane series of experimental aircraft. The X-15 set speed and altitude records in the 1960s. As of 2014, the X-15 holds the official world record for the highest speed ever reached by a manned, powered aircraft. Its maximum speed was 4,520 miles per hour (7,274 km/h).
X-24B - The X-24B made two precise landings on the main concrete runway at Edwards which showed that accurate unpowered reentry vehicle landings were operationally feasible. Pilots on these missions would fly steep descents and then perform a "flare out" maneuver at high speeds to make a 200 mph landing. These missions represented the final milestone in a program that helped write the flight plan for the NASA Space Shuttle program. source: Wikipedia
NASA's Supersonic Passenger Planes of the Future: Coming in 2025