The T-40 was a significant improvement over pre-war Soviet amphibious tanks and entered service shortly before the German invasion. The design was significantly different to that of its predecessors, the T-37 and T-38, and was characterized by an independent torsion bar suspension, welded construction and a turret fitted with a 12.7mm DShK machine gun. Amphibious capability was provided through a single propeller and steered by two rudders in the rear. It was more water-tight than previous amphibious tanks and this allowed it to cross larger rivers like the Dniepr and Dnestr. Unfortunately, despite the increase in armor protection (and consequently weight), the T-40 was no less vulnerable to small arms and shell splinters and its use as a tank during the desperate initial months of the war against Germany resulted in significant losses. It's machine gun, despite being capable of firing armor-piercing rounds, was also clearly inadequate against enemy tanks. As a result, production soon switched to the superior T-60 tank and it was largely retired from the battlefield by 1942.

Trials of the prototype T-30A, designed by N. A. Astrov took place between July-August 1939 and entered production as the T-40 in December of that year. A non-amphibious prototype known as the T-30B was also produced as the T-40, and it was given priority over its amphibious variant.