The T-40 had a bulky simple design and replaced T-37/38.
The early version had a 12.7 mm machine gun and the later a 20 mm. The first tanks was used in Finland 1940 and showed to be very ineffective being knocked out by small arms fire, and therefor the armor was thickened. Version A from late1941, had a bow top faired away at the sides which were originally flat. In 1942: the S- version, whose hull and turret armor was thickened and this resulted in a loss of amphibious capacity.
The T-40 project was soon abandoned for the T-60.