 was given the task of strategic air defense of the USSR. In the decades after World War II, this meant not only dealing with accidental border violations, but more importantly defending the vast airspace of the USSR against US reconnaissance aircraft and strategic bombers carrying free-fall nuclear bombs. The performance of these types of aircraft was steadily improved. Overflights by the very high altitude American Lockheed U-2 in the late 1950s revealed a need for higher altitude interceptor aircraft than currently available.3
The subsonic Boeing B-47 Stratojet and Boeing B-52 Stratofortress strategic bombers were followed by the Mach 2-capable Convair B-58 Hustler, with the even faster North American B-70 Valkyrie on the drawing board. A major upgrade in the PVO defence system was required, and at the start of 1958 a requirement was issued for manned interceptors capable of reaching 3,000 km/h and heights of up to 27 km (88,583 ft). Mikoyan and Sukhoi responded.4
The Mikoyan-Gurevich OKB had been working on a series of interceptors during the second half of the 1950s: the I-1, I-3U, I-7U, I-75, Ye-150, Ye-150A, Ye-152, Ye-152A, Ye-152P, and Ye-152M. The Ye-150 was noteworthy because it was built specifically to test the Tumansky R-15 engine, two of which would later be used for the MiG-25. This led to Ye-152, alternatively known as Ye-166, which set several world records.5 The Ye-152M (converted from one of the two Ye-152 aircraft) was intended to be the definite heavy interceptor design. But before it was finished, the PVO had selected the Tupolev Tu-128. As the work on the MiG-25 was well under way, the single-engine Ye-152M was abandoned.
Work on the new Soviet interceptor that became the MiG-25 started in mid-1959,6 a year before Soviet intelligence learned of the American Mach 3 A-12 reconnaissance aircraft.7 It is not clear if the design was influenced by the American A-5 Vigilante.6
The design bureau studied several possible layouts for the new aircraft. One had the engines located side-by-side, as on the MiG-19. The second had a stepped arrangement with one engine amidships, with exhaust under the fuselage, and another in the aft fuselage. The third project had an engine arrangement similar to that of the English Electric Lightning, with two engines stacked vertically. Option two and three were both rejected because the size of the engines meant any of them would result in a very tall aircraft which would complicate maintenance.6
The idea of placing the engines in underwing nacelles was also rejected because of the dangers of any thrust asymmetry during flight. Having decided on engine configuration, there was thought of giving the machine variable-sweep wings and a second crew member, a navigator. Variable geometry would improve maneuverability at subsonic speed, but at the cost of decreased fuel tank capacity. Because the reconnaissance aircraft would operate at high speed and high altitude the idea was soon dropped. Another interesting but impractical idea was to improve the field performance using two RD36-35 lift-jets. Vertical takeoff and landing would allow for use of damaged runways during wartime and was studied on both sides of the Iron Curtain. The problem has always been that engines dedicated to vertical lift do not contribute with any power in horizontal flight, and occupy space in the airframe needed for fuel. The MiG interceptor would need all the fuel it could get, so the idea was abandoned.8
The first prototype was a reconnaissance variant, designated "Ye-155-R1", that made its first flight on 6 March 1964.9 It had some characteristics that were unique to that prototype, and some of these were visually very evident: the wings had fixed wingtip tanks (600 litre capacity) to which small winglets were attached for stability purposes, but when it was found fuel sloshing around in the tanks caused vibrations they were eliminated. The aircraft also had attachments for movable foreplanes, canards, to help with pitch control at high speed (provisions for canards had previously been installed, but no