The T-34 was a Soviet medium tank produced from 1941 to 1958. It is widely regarded as having been the world's best tank when the Soviet Union entered World War II. The T-34 was developed from the BT series of fast tanks and was intended to replace both the BT-5 and BT-7 tanks and the T-26 infantry tank in service. The design and construction of the tank were continuously refined during the war to improve effectiveness and decrease costs, allowing steadily greater numbers of T-34s to be fielded. In early 1944, the improved T-34-85 was introduced, with a more powerful 88mm gun and a three-man turret design. By the war's end in 1945, the versatile and cost-effective T-34 had replaced many light and heavy tanks in service.