The Polikarpov I-3 () was a Soviet fighter designed during the late 1920s. It entered service in 1929, but was retired in 1935 with the advent of fighters with higher performance. Design and development Development of the I-3 began in mid-1926 after investigations into the loss of the Polikarpov DI-1 were completed. Although the new biplane shared many of the characteristics of the earlier design, including the staggered sesquiplane layout of the wings, it was a new design. It was designed by the OSS ( — Landplane Department) of Aviatrest (Aviation Trust) under the supervision of Nikolai Nikolaevich Polikarpov, head designer of the department. There was much debate within the OSS about the proper powerplant for the new fighter, but Polikarpov rejected the Wright Tornado radial engine and decided in favor of the BMW VI liquid-cooled V12 engine. A wooden mock-up was completed in April 1927, but formal approval of the design did not come until 3 June 1927. Static tests of a full-sized model began in October at the same time as negotiations for a license for the BMW engine were finished. The I-3 had an oval-section semi-monocoque fuselage covered with 'shpon', molded birch plywood, with a small headrest faired into the fuselage, although the engine was enclosed in a metal cowling. The two-spar wings were covered in plywood and fabric and had a Clark Y profile. Internal bracing wires were fitted to reinforce the wings. The control surfaces were framed in duralumin, but covered in fabric. It was provided with differential Frise-type ailerons. The duralumin N-type struts that separated the wings, and attached the upper wing to the fuselage, had a teardrop profile. They were reinforced with steel bracing wires. The conventional undercarriage was fixed with rubber shock absorbers and the tailskid was made from duralumin. The main gear could be replaced by skis like those fitted to the Polikarpov R-1. The engine's semi-retractable radiator extended below the fuselage behind the rear main gear struts. Two fuel tanks were fitted, the main one in the fuselage, but a small tank, mainly used to start the engine, was in the center section of the upper wing, along with the engine coolant tank. A total of of fuel was carried. Initially the I-3 was fitted with two fixed synchronized Vickers machine guns, but these were later replaced by PV-1 machine guns. A central OP-1 optical gunsight was provided with a