The airplane first set speed records in the Mach 4-6 range with Mach 4.43 on March 7, 1961; Mach 5.27 on June 23, 1961; Mach 6.04 on November 9, 1961; and Mach 6.7 on October 3, 1967. It also set an altitude record of 354,200 feet (67 miles) on August 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. The highly successful program contributed to the development of the Mercury, Gemini, and Apollo piloted spaceflight programs as well as the Space Shuttle program. The program’s final flight was performed on October 24, 1968.
http://history.nasa.gov/x15/cover.html
Mach 6.7 on October 3, 1967 ... that was 48 years ago ...
Today??
X-15 is not airbreathing at hypersonic. Not really a valid comparison.
I always liked the X-15 story about how Neil Armstrong bounced the plane off the atmosphere and almost had to do an emergency landing at LAX.
Altho I believe the part where he rolls the aircraft over to try to visually find a landmark through the tiny window and sees the Rose Bowl stadium and not the dry lake bed is apocyphal.