WWII Soviet Fighter-Biplane (winter version) The improved version of I-15 fighter was the I-15bis. Its main differences were “normal” wing centre section and NACA type engine cowling. Serial production of I-15bis began in 1937 and conti-nued to 1939. I-15bis were supplied to Spanish Republic (there they became nicknamed “Super Chatos”) and China. These fighters took part in air battles over Khalhin-Gol River, during Soviet-Finnish war (1939-1940). In the early stage of war in the Soviet-German front I-15bis biplanes were obsolete but used as fighters and ground attack aircraft.