The BT-5 ("Bystrohodnyi tank" or "Fast Tank type 5") was the second tank in the Soviet BT series of tanks. The BT-5 improved on the previous BT-2, such as a new turret fitted with a 45 mm anti-tank gun that was also used on the T-26 and the BT-5's younger brother, the BT-7. The BT-5 had a crew of three: a commander/gunner, a loader, and a driver. Both the commander/gunner and loader sat in the turret, and the driver sat in the front of the hull. The turret was a cylinder shape and some tanks had radio antennas around the turret. and the hull was riveted together with the front part shaped like a truncated pyramid.
The next time the BT-5 saw combat was the Soviet invasion of Poland. In the joint Soviet-German Invasion of Poland, where they did not see very much action. The first real challenge for the BT-5 was the Winter War against Finland, where the BT-5 was unable to get past the Finnish defensive line, the Mannerheim Line, where their thin armor was easily penetrated by Finnish anti-tank guns. The Soviets lost many BT-5s during this conflict. Finnish soldiers used Molotov cocktails and threw them at a weak point near the engine, which, when hit, would cause the engine to catch fire and explode.
After the Winter War, there were still hundreds of BT-5s in service, but many were replaced with the BT-7 and other tanks. On June 22, 1941, Nazi Germany launched Operation Barbarossa, the invasion of the Soviet Union, which caught the Soviets off guard. Soviet losses, including tanks, were appalling at the beginning of the invasion. The BT-5 would be used throughout the rest of the war, but not as much as its younger brother, the BT-7, or the new T-34, due to a lack of spare parts for the tank. The BT-5 was used the most at the beginning of the invasion of the Soviet Union and near the end of the war when spare parts were available.