Based on a mixed force of foreign tanks and imported prototypes, the Soviets developed an impressive domestic design and production capability. The T-26 light tank was based on the Vickers E (as were many other tanks of the period), chosen after it beat a Soviet FT derivative in trials. The Soviets purchased some U.S. Christie M1930 tank prototypes, from which they developed the BT series of fast tanks. They also developed the heavier multi-turreted T-28 medium tank and the massive T-35, which followed the design premise of the Vickers A1E1 Independent. Of the tanks produced between 1930 and 1940, 97% were either identical copies of foreign designs, or very closely related improvements. Significantly, the major improvement the Soviet designers made to these foreign designs was an increase in firepower. By 1935, the Red Army ". . . possessed more armoured vehicles, and more tank units than the rest of the world combined." (Zaloga 1984, p. 107)