It was found that the BT-2 wasn't adequate during the war with Finland so new requirements were devised for a vehicle with stronger armament, a single turret with the main gun and coaxial MG, space for a radio, and better mobility with an engine manufactured in the Soviet Union. The BT-5 was the design that was selected to meet those specifications.
It was the first main production model of the BT series.
When on wheels, steering was done by a steering wheel operating the front wheels. When on tracks, steering was done with the clutch and brake.
The turret was riveted with an extension to the rear to make room for the 45 mm model 1932 main gun and a 7.62 mm DT MG.
The 45 mm gun had a velocity of 2,350 ft/sec. The range it could fire was 4,800 meters. The velocity of an 1.43 kg shell was 760 m/sec. 42 mm of armor could be penetrated at 100 meters.
It had a larger turret, 2 rectangular escape hatches in the turret, new engine, and a stronger suspension.
The top of the turret a PTK panoramic scope.