Heinkel HD 37

The Heinkel HD 37 was a fighter biplane designed in Germany in the late 1920s but produced in the Soviet Union for Soviet Air Force service. It was a compact, single-bay biplane with staggered wings of unequal span braced by N-type interplane struts. Heinkel designed it for the clandestine air force the Reichswehr was training at Lipetsk, but German officials rejected it and bought the Fokker D.XIII instead. The Soviet Air Force, short of a successor to the Polikarpov I-5, tested two prototypes in Moscow in early 1928 with mixed results, and in early 1930 the Soviet government bought a licence to build it for three years, paying Heinkel 150,000 Marks. Built by TsKB and Polikarpov as the I-7 with a Mikulin M-17F engine, it first flew in summer 1931 and 131 were produced by 1934, serving briefly in Belarus before becoming obsolete.