In the period following World War 1, there came a global military movement centering on more cost-effective, light-class combat tanks. For the Soviet Union, established in 1922 after a lengthy and bloody civil war, there proved an additional quality of its light tanks that was sought - amphibious capability - and from this came the T-37 and T-38 series of light tanks appearing during the early- and mid-1930s. In 1938, Plant No 37 of Moscow began design of a 6.5-ton light scout tank to eventually replace the limited-capability T-37/38 series in the same role. Work proceeded on three pilot vehicles, one fitting a torsion bar suspension system and the other two with spring suspension, and these vehicles were made ready for testing in 1939.
During evaluations, the tanks underwent consistent changes to help improve their design. The torsion bar model became the favorite design and, by the end of it all, had received notable modifications to its hull shape, suspension system and powerplant. From this, the vehicle was showcased to Red Army authorities and promptly adopted for service as the "T-40" on December 19th, 1939. Plant No 37 was charged with its production which began in 1940 and ran until 1941. 151 had been produced by the time of the German invasion of the Soviet Union in June of 1941 and a further 41 were added to stocks in that month alone. However, only about 18 of the vehicles were actually deemed ready for combat service in a frontline capacity. Post-June models were then stripped of their amphibious capability to reduce manufacture times and these carried the designation of "T-40S" to signify their difference - July through September production netted 136 of these "land-only" tanks.
The T-40 was crewed by two personnel, a driver and the vehicle commander (who doubled as the gunner). Primary armament was a 12.7mm DShK Mod. 1938 heavy machine gun supplemented by a 7.62mm DT machine gun installation in a coaxial arrangement. The machine guns were fitted side-by-side in the front turret facing, the heavy gun to the right and the medium gun to the left. 500 rounds of 12.7mm ammunition were carried for the DShK and 2,060 rounds of 7.62mm ammunition were carried for the DT. The vehicle measured a length of 4.1 meters with a width of 2.3 meters and a height of 1.9 meters. Conditions were expectedly cramped internally but nonetheless suitable for its crewmembers who could readily switch positions if called to. The fighting compartment was set at the center of the hull and slightly offset from centerline to the left. The powerpack took up the right side of the hull. Water capabilities were accomplished through natural buoyancy of the hull for the upper half of the vehicle was purposely designed larger than the lower. An integrated propeller drive allowed for forward movement in water and dual rudders were applied for control. The turret was granted a circular access hatch along its roof while the driver was given a rectangular hatch above his position. The turret allowed for a full 360-degree traversal and limited engagement elevation from -8 to +25 degrees.
Outwardly, the T-40 exhibited a rather stout and squat appearance. It did continue the growing Soviet understanding of sloped, rolled armor plating to provide for basic ballistics protection. Final construction of these plates included both welding and riveting and some surface plates were removable for maintenance/repair access to key components (such as the engine compartment). Armor protection ranged from 4mm to 13mm, the thickest values across the more vulnerable, usually combat-facing surfaces.