Tupolev Tu-126 Moss, which is a modified variant of the Tu-54 Bear bomber. The design of the Mainstay was based on the Ilyushin Il-76, but with a lengthened fuselage. The Mainstay is considered the direct counterpart to the American Boeing E-3 Sentry AWACS aircraft.
To accommodate the large radar antenna with a rotating radome of ten metres diameter, the whole aircraft had to be refined. But also the interior of the aircraft had to be completely redesigned, in order to make room for the crew of the aircraft.