The wedge-shaped vertical stabilizer necessary for directional stability, upper speed brakes and a small tail-skid in retracted position is visible on the first X-15 on display at NASM. Just three of these hypersonic aircraft were built, and only 12 pilots flew them including Neil Armstrong for a total of 199 flights from 1959-68. The X-15 set the world's unofficial speed record of Mach 6.70 with Flight No. 2-53-97, and an altitude record of 354,200 ft. with Flight No. 3-22-36. The second X-15 is displayed at NMUSAF. The third X-15 was lost on 15 Nov 1967 on Flight 3-65-97 in a fatal accident.