In 1925, Kalinin made a series of single-engined aircraft
characterized by having a quasi-elliptical wing that made his OKB famous
in the USSR. In 1930, he designed a gigantic transport aircraft, the K-7
which has seating capacity of 60 or 20 tonnes of cargo. The K-7 design
caused attention from the Ministry of Aviation Industry. It was given
permission for the prototype to be built, but with the role changed to a
heavy bomber. The aircraft was completed and was taxied onto the runway
in June 1933. When all the seven engines were run a serious vibration
throughout the aircraft was noted, so the affected parts were quickly
strengthened. In August 1933 the K-7 made the first flight. Continuous
tests revealed fair flying characteristics. In November 1933, during
speed runs at low altitude, resonance suddenly struck and the right tail
boom fractured, The K-7 dived into the ground and burned. The USSR
aviation leadership changed views on production of heavy aircraft.
Kalinin was not allowed to complete the development of K-7. The scheme
was abandoned in 1935.