The Tupolev ANT-40, also known by its service name Tupolev SB ( – Skorostnoi Bombardirovschik – high speed bomber) and development co-name TsAGI-40, was a high speed twin-engined three-seat monoplane bomber, first flown in 1934. The Tupolev design was advanced but lacked refinement, much to the dismay of crews, maintenance personnel, and Stalin, who pointed out that "there are no trivialities in aviation". Numerically the most important bomber in the world in the late 1930s, the SB was the first modern stressed skin aircraft produced in quantity in the Soviet Union and probably the most formidable bomber of the mid-1930s. It was produced in the Soviet Union and was also built under license in Czechoslovakia. Many versions saw extensive action in Spain, the Republic of China, Mongolia, Finland and at the beginning of World War II against Germany in 1941. It was also used in various duties in civil variants, as trainers and in many secondary roles. Successful in the Spanish Civil War because it outpaced most fighters, the aircraft was obsolete by 1941. By June 1941, 94 percent of bombers in the Red Army air force (VVS RKKA) were SBs. Development In 1933 the Soviet Air Force ministry (UVVS) issued an outline requirement for a high-speed bomber. Work on this proposal at TsAGI began in January 1934. The SB was designed and developed in the Tupolev KB ("Design Bureau") by a team led by A. A. Arkhangelski. Two versions were planned – with Wright Cyclone radial engines (ANT-40 RTs), and with the Hispano-Suiza 12Y liquid-cooled V12 engines (ANT-40 IS). The skills gained in the design of the MI-3 and DI-8 aircraft were widely used. The first two prototypes were designed as ANT-40.1 and ANT-40.2. The Cyclone powered prototype flew first, on 7 October 1934, while the first Hispano-Suiza powered prototype (ANT-401), which featured a larger wing, flew on 30 December 1934, demonstrating superior performance. The second Hispano-Suiza powered aircraft, the ANT-402 was considered a production prototype, and its performance was impressive. It was however plagued by teething problems, leading unhappy test personnel to cover the ANT-402 with placards listing the aircraft's defects prior to a visit by Sergo Ordzhonikidze, the Commissar for Heavy Industry. On seeing these placards, Ordzhonikidze summoned Tupolev to a meeting at the Kremlin to discuss these shortfalls. When Tupolev stated that most of the defects were trivial, Joseph Stalin said: The first production aircraft, designated