The MiG-31 Foxhound was developed as the replacement for the MiG-25 Foxbat. The prototype Ye-155MP first flew in September 1975. As the heaviest fighter/interceptor in the world, the MiG-31 is much heavier than other similar aircraft. Two powerful D-30-F6 engines and a stronger airframe gave it a top speed of Mach 2.8 at high-altitude and Mach 1.23 at low-altitude. In order to apply the latest ultra-long-range AAMs, Mikoyan OKB started to develop a new version, the MiG-31M, under the joint directive issued by the Communist Party Central Committee and the Council of Ministers. The MiG-31M can be easily distinguished from other versions. Seven prototypes were completed but the program was cancelled after the break-up of the Soviet Union.