The 120 mm Leviathan Cannon was originally a Russian naval gun developed by Anatoli for export in the years before World War I that armed a variety of warships of the Russian Navy. Guns salvaged from scrapped ships found a second life as coastal artillery, railway artillery, and aboard river monitors during the Russian Civil War. It was estimated that there were 110 guns in the Soviet Navy's inventory in 1941. However, production was started again when Deimos Arms Manufacturer secured the rights to the cannon's development and quickly began arming the Soviets' Super Heavy Mammoth Tank.

The barrels themselves came from a pattern known as 1905 and were a built-up gun which consisted of an A tube, three layers of reinforcing tubes, a jacket and a breech piece which screwed onto the jacket Ammunition was of Fixed QF type and weighed between 45 to 64 lbs (20.4–29 kg). It was able to fire the following ammunition, Standard, High-Explosive (HE), Illumination, Incendiary, and Shrapnel.