these types of aircraft was steadily improved. In the late 1950s, the very high altitude overflights of Soviet territory by the Lockheed U-2 revealed the need for a higher altitude interceptor aircraft than available at that time. The subsonic Boeing B-47 Stratojet and Boeing B-52 Stratofortress strategic bombers were followed by the Mach 2 Convair B-58 Hustler, with the Mach 3 North American B-70 Valkyrie on the drawing board at that time. 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 and heights of up to . Mikoyan and Sukhoi responded. 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. 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. Development Work on the new Soviet interceptor that became the MiG-25 started in mid-1959, a year before Soviet intelligence learned of the American Mach 3 A-12 reconnaissance aircraft. It is not clear if the design was influenced by the American A-5 Vigilante. 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. Options two and three were both rejected because the size of the engines meant that either of them would result in a very tall aircraft, which would complicate maintenance. 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.