Designed in 1930, the Kalinin K-7 heavy experimental aircraft was an unique plane. With a large wingspan close to that of a B-52 and a much greater wing area, the K-7 was one of the biggest aircraft built before the jet age. Powered by six tractor engines on the wing leading edge and a single engine in pusher configuration at the rear. The K-7 could attain speeds of up to 21 knots (140 mph).
One of the most striking features of the plane were the large underwing pods. These housed the landing gear along with machinegun turrets, with room inside each for several people. The area inside the wings was also physically accessible with enough space to arrange passenger seats lengthways.
The K-7 first flew on 11 August 1933. There were initial issues with vibration and instability, after these were resolved the aircraft saw seven successful test flights before a crash due to a structural failure of one of the tail booms.