In the 1950s, the Soviet Union faced the task of defending its borders against intrusions by Western spy planes and deterring an attack by Western bomber armadas. The mission was to intercept incoming adversaries at long range, which required plenty of engine power, ample fuel and heavy armament. Several Soviet design bureaus took on this priority task. The first heavy interceptor from the Mikoyan stable was the swept-wing I-3 of 1956 which, though never flown, was the precursor of the cannon-armed I-7U and the missile-armed I-75. These were followed by the delta-winged Ye-150 series of the late 1950s and early 1960s - the single-engined Ye-150 and Ye-152 and the twin-engined Ye-152A. Known to the outside world as the Ye-166, the huge Ye-152M set an impressive speed record of 2,681 km/h on 7th July 1962. These aircraft paved the way for the MiG-25 interceptor. Other contenders included the Sukhoi T-37, terminated by government order before it had a chance to fly. Even the Tupolev OKB