Between 1976 and 1988 production began on the first series of the baseline MiG-31. There were 519 aircraft produced during this period at the Sokol factory and 40 of these aircraft would later be upgraded to MiG-31BS. The next series ran from 1989 to 1991 and consisted of 101 MiG-31DZs followed by the final batch of 69 MiG-31Bs between 1990 and 1994. The Kazakhstan Air Force retained 50 MiG-31Bs after the breakup of the Soviet Union. The MiG-31B received upgraded avionics and an in-flight refueling probe and new R-335S missiles.