R-40 (missile)
The Bisnovat (later Molniya then Vympel) R-40 (NATO reporting name AA-6 'Acrid') was a long-range air-to-air missile developed in the 1960s by the Soviet Union specifically for the MiG-25 interceptor, but can also be carried by the later MiG-31. It is the largest air-to-air missile in the world to ever go into production.
Development
The development of the Mach 3+ North American XB-70 Valkyrie threatened to make the entire interceptor and missile force of the Voyska PVO obsolete at one stroke, thanks to its incredible speed and altitude performance. In order to counter this new threat, the MiG-25 was designed, but new air-to-air missiles were also required to enable the MiG-25 to engage its intended targets at the high speeds and altitudes dictated by the requirements. The Bisnovat design bureau began development of the long-range air-to-air missile in 1962. The resulting R-40 was initially matched with the Smerch-A ("Tornado-A") radar of the MiG-25. It was built in semi-active radar homing (R-40R) and Infrared homing (R-40T) versions.