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The sign which accompanied the X-15.

It read

NORTH AMERICAN X-15
World's Fastest and Highest Flying Aircraft

The North American X-15, a rocket-powered research aircraft, bridged the gap between manned flights in the atmosphere and space flight. After its initial test flights in 1959, the X-15 became 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).

The X-15 was designed to explore the problems of flight at very high speeds ant altitudes. It was carried to an altitude of 12,000 meters (40,000 feet) under the wing of a Boeing B-52 bomber. During one test, it attained an altitude of over 108 kilometers (67 miles), flying so high that it functioned more as a spacecraft than an airplane. In 1967 it reached Mach 6.72 (7,297 kilometers or 4,534 miles per hour).

Transferred from the National Aeronautics and Space Administration

Wingspan:	6.7 m (22 ft)
Length:	15.5 m (51 ft)
Height:	3.9 m (13 ft)
Weight, gross:	17,237 kg (38,000 lb)
Engine:	Thiokol (Reaction Motors) XLR-99-RM-2 rocket engine
Manufacturer:	North American Aviation

The X-15 is NASM catalog #A19690360000.