After the tank made its first appearance on the battlefield during WWI, tank design in Europe evolved with faster, better armored, and deadlier tanks. However, U.S. tank design and doctrine development stagnated during the interwar period. When WWII broke out, America’s armored corps severely lagged behind Germany’s. Seeing the effectiveness of Panzer IIIs and Panzers IVs in France, the U.S. Army ordered a new medium tank with a 75mm gun: the M4 Sherman. In the meantime, the M3 Medium Tank was the best that America had.
The Lee needed a crew of seven while the Grant needed six (U.S.