Aleksandr Ivanovich Putilov designed fuselages and landing gears under A.N. Tupolev at CAHI, but was enticed away 1928 to head Stal (steel) group at VVA academy, which became OOS (section for experimental aircraft construction) 1930. Produced Stal-2 high-wing 6-seater (11 October 1931), improved Stal-3 (1933), Stal-5 18-passenger flying wing with 2 x 900-hp M-34F (late 1935) and Stal-11 high-speed transport (autumn 1936), all with steel airframes.