the outbreak of wwii in europe caused the u. S. Army to recognize it was lacking an effective tank force. Design of the medium tank model 3 commenced in july 1940, with production beginning in december, and the first units became operational in late 1941. The u. S. Army required a medium tank armed with a 75mm main gun and the united kingdom had made an immediate demand for more medium tanks. The odd design archaically mounted the 75mm main gun in the right sponson, with another 37mm gun mounted in a small turret on top, which was a compromise meant to produce a tank as soon as possible until the m4 became available. In britain, the tank was called by two names based on the turret configuration and crew size. Tanks fitted with the us style turrets were called the lee", named after confederate general robert e. Lee. Variants using british style turrets were known as the "grant," named after union general ulysses s. Grant. The m3 offered potent firepower and sturdy armor, but had serious shortcomings in its general design and shape, including a high silhouette, and the sponson mounting of the 75mm main gun, which limited traverse and prevented the tank from taking a hull-down position. It also featured riveted construction (creating deadly internal projectiles with non-penetrating hits), and limited off-road performance. Multiple variants and derivatives were spawned from m3 production, including the m31 armored recovery vehicle, m7 priest self propelled gun, and even the canadian ram cruiser tank. "