The BT were the basic AFV of independent tank brigades, and of mechanized and cavalry divisions. They were intended for use in the long-range exploitation role. Developed from the M1931 design of the US Christie M1931 “race tank”, the BT could be driven on their wheels, sans tracks—although this theoretical advantage was rarely used. BT were well-liked by their crews, but their thin armor made them vulnerable to even ATR. Almost 5,000 BT were built.
The BT-5 (Bystrochodnij Tankov or “Fast Tank” type 5) was an improvement on previous models. The BT-5 shared with its predecessors the suspension, the hull (though slightly reinforced) and equipment. The turret was new, roomier and fitted for the first time with the standard 45 mm gun, also used on the T-26.
Aside from the turret, the BT-5 could hardly be distinguished from its predecessor. The hulls were identical, with the same internal arrangement and the same Mikulin M5 engine, supplying 400 hp. The armor was unchanged, ranging from 6 to 13 mm, in no way capable of withstanding even antitank rifle fire. Speed was seen as active protection. The big difference was the turret, similar to the model 1933 T-26 two-men turret with a 45 mm model 32 gun, the new standard Soviet AT gun.
Commander tanks, had a radio 72. A DT-5 coaxial machine-gun completed the armament. The gun had a single episcope. There was no commander cupola, just simple escape hatches. The turret itself increased the overall weight to 11.5 tons, and performance degraded accordingly. Top speed was 72 km/h instead of 100 km/h. The fuel tank shrank to 360 liters from of 400. The engine now gave a 35 hp/ton ratio and maximum range was decreased from 300 to 200 km (tactical range 90 km). But all this was sacrificed for largely improved firepower.
Like the previous model, production was assumed by KhPZ (Kharkov). Early models were provisionally fitted with an enlarged cylindrical turret (single hatch, with or without basket) and BT-2 heavy steel road wheels. But soon after this first series, UMM instructed the design teams in Leningrad and Kharkov to adapt their model to a common turret. Thus the early model 1933 was introduced, criticized later for its awkwardly placed hatch and inadequate stowage.
The road wheels were replaced by new, lighter convex models in 1933, which were used to upgrade the BT-2 as well. By 1934, the OKMO team had developed a better turret, with a large bustle and twin hatches, also shared with the T-26 until 1937. These were fitted to already existing vehicles . The commander tanks could be distinguished by their turret horseshoe antenna and 71-TK-1 radio set.
The BT-5 entered service in 1933 and, with gradual deliveries until 1935, equipped all armored cavalry brigades. The first active engagements came in Spain, in 1937, when a batch of 100 was shipped to the Republicans. They took part in the defense of Madrid. Some were captured later and saw service with the Nationalist Spanish state. Two brigades (6th and 11th Tank Brigade) were sent in the Far East, on the Sino-Russo-Mongolian border, to face Japanese incursions of the time. They proved instrumental at Khalkin Gol, with many BT-7s, proving too fast for the Japanese AT teams and still deadly for any AVFs deployed. However, they proved vulnerable to “close quarter” Japanese teams armed with Molotov cocktails.
Not long after, they were deployed in eastern Poland. They also soldiered during the “Winter War” in Finland, proving ill-adapted for the task with their thin armor. Losses were appalling. Finnish troops used Molotov cocktails as well and quickly found a weak point where the engine was installed, prone to catch fire and explode when hit, as shown in reports. In 1941, there were still hundreds of BT-5s in service despite the type having been replaced by the BT-7. But hundreds were lost or abandoned, worn out, during the summer offensive, liquidating what was left of the model. Only the lack of spare parts prevented the use of surviving vehicles until later in the war.