The early model 1933 was introduced, criticized later for its awkwardly placed hatch and inadequate stowage. The roadwheels were replaced by new, lighter convex models in 1933, which were used to upgrade the BT-2 as well. By 1934, 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.

This first series was equipped with an adapted version of the T-26 turret, shared with the BT-5. This had a 45 mm (1.77 in) 20K gun with a coaxial DT-model machine-gun. The crew of three was unchanged and virtually all other components were similar to the BT-5, most importantly the suspension, rubberized roadwheels, drive sprockets, idlers and tracks. The engine was also the same petrol radial Mikulin M-17T (V-12) derived from a licence-built BMW. Steering was performed with a control stick and the engine had three forward and one reverse gears.