This is X-15A #1, Air Force serial number 56-6670. X15 #2 (56-6671) was rebuilt following a landing accident as the advanced X-15A-2. It was the fastest X-15 flown and is now on exhibit at the U.S. Air Force Museum (see the Presidential and R&D portion of the National Museum of the United States Air Force section of this Website). The X-15 #3 (56-6672) crashed during an atmospheric reentry.
North American
X-15A-1
The X-15 was the first winged aircraft to attain hypersonic velocities of Mach 4, 5, and 6 (four to six times the speed of sound) and to operate at altitudes well above 30,500 meters (100,000 feet). Three X-15 research aircraft were built and flown, completing a total of 199 research flights.