North American Aviation manufactured 3 airframes. They flew a total of 199 flights between 1959-1968. This joint program by NASA, the Air Force, the Navy, and North American operated the most remarkable of all the rocket research aircraft. Composed of an internal structure of titanium and a skin surface of a chrome-nickel alloy known as Inconel X, the X-15 first set speed records in the Mach 4-6 range with Mach 4.43 on Mar. 7, 1961; Mach 5.27 on June 23, 1961; Mach 6.04 on Nov. 9, 1961; and Mach 6.7 on Oct. 3, 1967. The airplane also set an altitude record of 354,200 feet (67 miles) on Aug. 22, 1963, and provided an enormous wealth of data on hypersonic air flow, aerodynamic heating, control and stability at hypersonic speeds, reaction controls for flight above the atmosphere, piloting techniques for reentry, human factors, and flight instrumentation of relevance not only to aeronautics but to spaceflight.