Several excellent designs began production in 1940–41. Just prior to the war, the Red Army embarked on two closely related projects to reorganize its mechanized forces and re-equip them with modern designs that took lessons learned from the Spanish Civil War, the Battle of Khalkhin Gol and the Winter War into account. Some of these designs were copies of other countries' designs. The most significant was the T-34, which was originally designed as the successor to the BT series. Its heavier armour and dual-purpose gun, made it one of the best medium tanks of the first half of World War II. The T-34 eventually took on the roles of many other Soviet tanks. The T-34's chassis made it usable after the war, as it could be continuously upgraded with heavier guns, new turrets and other modifications. The other significant design was the KV-1 tank. These were armed with the same 76.2 mm gun as the T-34, and had the same Kharkiv model V-2 diesel engine. The KV tanks were equipped with a torsion bar suspension and much heavier armour than the T-34. The KV was slow, intended as a breakthrough tank. The KV-2 close-support version was armed with a howitzer. The KV series was the main Soviet heavy tank until 1943, with the end of production and the destruction of most surviving vehicles. Early in 1944, the KV's successor, the IS-2 was introduced. It was armed with a gun, with thicker armour and better mobility. A new infantry-support tank was introduced in 1941: the T-50. It was intended to be the replacement for the T-26, and was equipped with a 45 mm gun, torsion-bar suspension and thicker armor than most other tanks in its class. However, production problems with its new engine led to the tank being cancelled after fewer than 70 had been made. The light reconnaissance role was be filled by the amphibious T-40 and the cheaper, non-amphibious T-60.