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 (Zaloga & Grandsen 1984:66, 111). At its introduction, it was the tank with the best balanced attributes of firepower, mobility, and protection in existence, although initially its battlefield effectiveness suffered from the unsatisfactory ergonomic layout of its crew compartment, lack of radios and poor tactical employment. The two-man turret crew arrangement required the commander to also serve as the gunner, an arrangement common to most Soviet tanks of the day; this proved to be inferior to the German arrangement of three men (commander, gunner and loader).
The design and construction of the tank were continuously refined during the war to improve effectiveness and decrease costs, allowing steadily greater numbers of tanks to be fielded. In early 1944, the improved T-34-85 was introduced, with a more powerful 85 mm 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, and accounted for the majority of Soviet tank production. Its evolutionary development would lead directly to the T-54/55 series of tanks, built until 1981 and still operated today.