The X-15 was a research airplane in the purest sense, whereas the XB-70 was an experimental bomber intended for production but diverted to research
Noteworthy these aircraft represent two different approaches to flight research. The X-15 (that still holds official world record for the highest speed ever recorded by a manned, powered aircraft) in fact was a research airplane in the purest sense, whereas the XB-70 (then the world’s largest experimental aircraft) was an experimental bomber intended for production but diverted to research when production was cancelled by changes in the Department of Defense’s offensive doctrine.
The X-15A-2 had been modified from its original configuration with a longer fuselage and drop tanks. To protect it against aerodynamic heating, researchers had coated it with an ablative coating covered by a layer of white paint. These changes allowed the X-15A-2 to reach a maximum speed of Mach 6.7, although it could be sustained for only a brief period.
The XB-70, by contrast, was designed for prolonged high-altitude cruise flight at Mach 3. The aircraft’s striking shape–with a long forward fuselage, canards, a large delta wing, twin fins, and a box-like engine bay–allowed it to ride its own Mach 3 shockwave, so to speak. A joint NASA-Air Force program used the aircraft to collect data in support of the U.S supersonic transport (SST) program, which never came to fruition because of environmental concerns.
Photo credit: NASA