The Mikoyan MiG-31 (Russian: Микоян МиГ-31; NATO reporting name: Foxhound) is a supersonic interceptor aircraft developed to replace the MiG-25 Foxbat. The MiG-31 was designed by the Mikoyan design bureau based on the MiG-25.
Like the MiG-25, MiG-31 is a large twin-engine aircraft with side-mounted air intakes, a shoulder-mounted wing, and twin vertical tail fins. Unlike the MiG-25, it has two seats, with the rear occupied by a dedicated weapon systems officer.
The most important development was introducing an advanced radar capable of both look-up and look-down engagement (locating targets above and below the aircraft), as well as multiple target tracking. This finally gave the Soviets an interceptor able to engage the most likely Western intruders at long range. It also reflected a policy shift from reliance on ground-controlled interception (GCI) to greater autonomy for flight crews.
The MiG-31 entered operational service with the Soviet Anti-Air Defense (PVO) in 1982. It was first photographed by a Norwegian pilot over the Barents Sea in 1985. The MiG-31 was sought after for a variety of long-range missions.
Approximately 370 are in Russian service, with another 30 or so in Kazakhstan. The MiG-31 will likely continue serving for years to come, depending on upgrades and future growth of the Russian economy.
Source: Wikipedia, "Mikoyan MiG-31", available under the CC-BY-SA License.