The first tank of this class were T-18 (also known as MS-1) which were developed and produced in 20th. Later, the T-19 and T-20 were intended to replace the T-18, but they were delayed because of trouble with their engines. Thus, those projects were stopped and instead of that, the British light tank "Vickers E" was produced under license (that doesn't mean it was a mass production, rather an attempt to develop a new tank on a "Vickers" base).
On 28 May 1929, an agreement with the Vickers corporation was signed and the first party of 15 tanks was delivered to the Soviet Union. Those tanks were named V-26. Their design became a basis for the further Soviet tank modernisation: TMM-1 and TMM-2. Extensive comparative tests of the B-26, T-19, T-20, TMM-1 and TMM-2 were carried out on the Kubinka proving ground.
The T-20 showed the best result, however it was equipped with an experimental engine, that wasn't in a mass production. So, on February 13 of 1931 the RVS signed an order to accept for Red Army service the "Vickers E" light tank under the new designation of T-26 and recommended to start its mass production.
The STZ was ordered to start manufacture these tanks (with a 13,800 vehicles of yearly output!) However this industrial plant was still under construction, thus another factory (the "Bolshevik" factory in Leningrad) received the same order. Later, "Bolshevik" was split into two independent factories - "Bolshevik" and #174 "K.E.Voroshilov" which continued to produce the T-26 tanks. Over 10 years this factory produced more than 12,000 tanks of that type. The last tanks were produced at the beginning 1941.
The first production T-26 tanks had two turrets and were similar to the original "Vickers." The first party of T-26's served in the Red Army from September 1931 and took part in a military parade on Red Square in November 7. The T-26 became prevalent tank in the Soviet tank forces. Many further projects were based on this tank.
However, Soviet tankers strongly disliked this tank. Although it was produced in large numbers, it had still many defects. For example, it had a thin armor and a weak engine. During 1931-1941, 23 modifications were made in attempts to improve its engine. However, engine power was increased from only 90 h.p. to 97 h.p. That was obviously insufficient.
During the first two years (1931-1932) the only twin-turret model of T-26 were manufacturing. Later, when various modifications appeared, the Red Army designated them as "T-26 model 1931-32", "T-26 model 1933-37" and "T-26 model 1938-39". The T-26A, T-26B, T-26C etc. designations were used by the Germans during the WWII and completely unknown in the USSR (Besides, T-26A means artillery tank based on regular T-26 but armed with short-barreled 76mm Main Gun). Each turret of the twin-turret version was armed with one machine-gun and could rotate on 270°.
Tanks armed with a gun were used as commander's tanks; that's why some of them were equipped with radio. 37mm gun was originally a Hochkiss gun, modified in the USSR. Later, it was replaced with the Soviet 37 mm PS-2 gun model 1930 (Designer - P.N.Syachentov). That was a fully redesigned Rheinmetall 37 mm AT-gun Pak 30 with a longer barrel and higher muzzle velocity. Tanks armed with those guns were known as the T-26 Model 1932.
Production of the T-26 with a single turret (the same turret as on BT-5) has started in 1933. That model was equipped with the a 45 mm gun 20K model 1932/38 as well as a DT TMG. That was the most numberous modification of T-26, over 5,000 tanks were built. During the production many changes were made: capacity of the fuel tanks was increased, tank received an antiaircraft machine-gun. After 1937, all tanks started to equip with a 71-TK-3 radio. A powerful searchlight was installed on the gun for a night combat. Different versions of the T-26 carried a various amounts of ammunition.
In 1938, the new T-26 version with the new welded conic turret was developed. The tank was armed with the newest 45 mm gun 20K model 1932/38, advanced gunnery optics and had an increased capacity of the fuel tanks.
The next modification - the T-26 model 1939 - was developed in 1939. That model had a sloped armor and improved engine ventilators that were protected against the "Molotov cocktails" (derived from the combat experience in Spain Civil war). After adding of those modifications, the tank weight increased. Up to end 1939, more than 1,300 tanks were produced.