Why was the MiG-25 designed?

In the late 1950s, flights by the Lockheed U-2 at very high altitude over Soviet territory revealed the need for an interceptor capable of operating higher than those available at the time. In early 1958 a requirement was issued for manned interceptors able to reach 3,000 km/h (1,600 kn) and altitudes of up to 27 km (89,000 ft). Work on the new Soviet interceptor that would become the MiG-25 began in mid-1959, a year before Soviet intelligence learned of the American A-12 reconnaissance aircraft, capable of Mach 3.

The subsonic Boeing B-47 Stratojet and B-52 Stratofortress were followed by the Mach 2 Convair B-58 Hustler, and at the time the Mach 3 North American B-70 Valkyrie was under development. The MiG-25 was conceived to intercept fast, high-altitude aircraft such as those from the F-108 Rapier program, the XB-70 Valkyrie, and, more importantly, the A-12, from which the SR-71 spy plane was derived. Some argue that the XB-70 threat was the main, and only, reason for the design, but the MiG OKB has stated that the underlying motive was to counter the A-12/YF-12 program. The B-70 Valkyrie, the Mach 3 American bomber the MiG-25 was meant to answer, was eventually cancelled.

The Foxbat was not designed for dogfighting: it was an interceptor meant to climb fast, fly fast in a straight line, fire missiles at high-altitude bombers, and return home. The first prototype, designated Ye-155-R-1, made its maiden flight on March 6, 1964.