As the Allied light tank was being designed, provisions were made so that multiple types of main armament could be mounted in the turret. The possibility of mounting the main gun of a heavy tank, and the 3-inch gun of an anti-armor cannon was explored first, but its size and weight made it too large to fit in the turret of the Allied tank. Development on a new 76 mm gun better suited to the light tank began in the fall.

Before the start of WWII, tests began on the feasibility of mounting a 105 mm howitzer into the turret of the tank. The basic 105 mm howitzer was found to be ill-designed for mounting in a tank turret, so it was completely redesigned and re-designated the 105mm howitzer. After modifications to the turret (concerning the balancing of the gun and the strength of the power traverse) and interior of the hull (concerning the stowage of the 105mm ammunition), the Ordnance Department expressed its approval of the project, and production of Allied light tanks armed with 105 mm howitzers began in February.

The light tank would enter combat equipped with the 75 mm gun, a 40-caliber gun that could penetrate an estimated 3.5 inches (88 mm) of rolled armor at 90 degrees, a range of 110 yards (100 m), and 2.9 inches (73 mm) at 1,100 yards (1,000 m) firing the usual armor-piercing round and equipped with a telescopic gunsight. Facing the early Soviet tanks, the light tank's gun could penetrate the frontal armor of these tanks at normal combat ranges, within 1,000 yd (910 m). Allied Intelligence discounted the arrival of the Soviet heavy tank and the Mammoth tank, predicting that the Mammoth tank would be a heavy tank like the others, and doubted that many would be produced. There were also reports of British anti-tank guns being able to destroy the Soviet heavy tanks. 

However, this only happened at very close ranges and against the thinner side armor. Due to their misconceptions related to this, and also due to tests that seemed to prove that the 76 mm gun was able to destroy both the Soviet heavy tank and the Mammoth tank, the leadership of Army Ground Forces were not especially concerned. The criteria and results of the 76 mm gun tests were later ruled to have been inaccurate when compared to real-world conditions, with military leaders even remarking that they were wrongly told by Ordnance that the 76 mm could knock out any Soviet tank. The Army also failed to anticipate that the Soviets would attempt to make the Mammoth tank the standard tank of their tank divisions, supported by small numbers of heavy tanks.