Ag1-4=Engines VTOL=bombbox Trim=Trim In 1925, the Soviet Air Force approached TsAGI with a requirement for a heavy bomber with total engine output of 1,500 kW (2,000 hp) and either wheeled or float landing gear. Tupolev OKB started design work in 1926 with the government operational requirements finalized in 1929.[2][3] The Tupolev TB-1 was taken as the basis for the design and the aircraft was initially powered by 440 kW (590 hp) Curtiss V-1570 "Conqueror" engines,[4] with the intent of switching to Mikulin M-17s (modified BMW VIs) in production.[2][5] mock-up was approved on 21 March 1930 and the first prototype was completed on 31 October 1930.[6] The aircraft flew on 22 December 1930 with Mikhail Gromov at the controls and with ski landing gear. Despite almost crashing owing to vibration causing the throttles to close, the test flight was a success.[2][4] On 20 February 1931, the Soviet Air Force approved mass production of the ANT-6 with M-17 engines.[6] The prototype was refitted with 540 kW (720 hp) BMW VIz 500 engines, larger radiators, and wooden fixed-pitch propellers of TsAGI design. Single-wheel landing gear was deemed too weak and was replaced by tandem bogies with 1,350 mm × 300 mm (53 in × 12 in) tires.[2] The first pre-production TB-3 4M-17 flew on 4 January 1932 with Andrey Yumashev and I. F. Petrov at the controls. Unexpectedly, subsequent mass-produced aircraft were found to be 10–12% heavier than the prototype, which significantly hampered performance.[2] The discrepancy was discovered to be due to high positive tolerances on raw materials which resulted in steel sheetmetal, pipes, and wires being much thicker than on the carefully constructed prototypes. The aircraft were also more crudely painted with a thick layer of camouflage and lacquer.