First of Tupelov’s designs to use smooth skin, this two-engined bomber underwent constant development. The Soviets modified if for use as a dive-bomber (the Ar-2) and a civil airliner (the PS-40 & PS-41). It served in the Soviet Air Force, Civil Air Fleet, and the Soviet Polar Aviation Directorate. It was supplied to the republican Air Force for use during the Spanish Civil War. The SB proved to be faster than the other bombers, and even faster than the fighters sent up to intercept it. Supplied to China in late 1930s, it was used against the Japanese through May 1943. Other examples were given to the Czechoslovakian Air Force and the design was later produced by the Czechs under license as the Avia B-17. The SB was used against Finland during the Soviet-Finland (Winter) War. Captured examples were used by the Finns, and the Germans also later supplied the Finns with some of their cache of captured SBs for use against Soviet submarines during World War II.