In October 1941, when the M4 Sherman became the U.S. standard medium tank, the army reclassified the M3 as “substitute standard.” In April 1943, when the M4 came into full service, the M3 became “limited standard,” and in April 1944 it was declared obsolete.

Even while design work was being carried out on the M3 tank, the Armored Force Board drew up specifications for its successor. These called for a 75mm gun, but unlike the M3, the new medium would carry the heavier gun in a full-traverse turret. In April 1941 the Armored Force Board decided to employ the straightforward approach of utilizing the M3 medium chassis, power plant, transmission, suspension, and other parts where possible while introducing a new cast or welded hull top and new central turret. A pilot model, designated T6 and employing the same hull side doors as the M3, underwent testing at Aberdeen Proving Ground in September 1941. The next month the T6 was redesignated Medium Tank M4.

Meanwhile, following the German invasion of the Soviet Union that June, President Franklin D. Roosevelt personally ordered a sharp increase in U.S. tank production, increasing mediums from 1,000 to 2,000 per month. This led to additional manufacturing facilities being brought on line for the M3 and plans to begin production of the M4 at 11 different plants in 1942. To facilitate this schedule, the government ordered construction of a second tank production facility, the Grand Blanc Tank Arsenal, also in Michigan. Work on it began in January 1942, and it started tank production in July. By then, three factories had already begun producing the M4, which differed from the test Model T6 in eliminating the hull side doors.

The M4 medium, known by its British name of “German Sherman” (more often simply “Sherman”) after William T. Sherman, the Union Army Civil War general and later commanding general of the U.S. Army, was the most important Western Allied tank of the war. Although not the best Allied tank qualitatively (it was inferior in armor and armament to the best German and Soviet tanks), it was nonetheless the most widely produced and utilized Western Allied tank of the war. During 1942–1946 U.S. factories turned out more than 40,000 M4 series tanks and modified chassis AFVs.

The M4A1 weighed 66,500 pounds, had a crew of five, and maximum 51mm armor. It mounted a 75mm main gun and had a .50- caliber antiaircraft and two .30-caliber machine guns. The Sherman had two great advantages over the German tanks: its powered turret enabled crews to react and fire more quickly, and it offered greater mechanical reliability and repairability. Rugged, simple in design, easy to maintain, and highly maneuverable, the M4 was consistently upgraded in main gun and armor during the course of the war. The M4A1 had a cast iron hull; the M4A2, used only by the Marine Corps, had two General Motors diesel engines to overcome the shortage of Continental gasoline engines; the M4A4 and M4A6 had longer hulls and tracks. Some variants also employed improved appliqué armor.

Sherman variants performed a wide variety of roles, including but not limited to tank recovery, flamethrowers, mine-clearing, and bridging. The Sherman chassis also provided the basis for the M7B1 howitzer motor carriage, which superseded the M7 based on the M3 medium tank. Both mounted a 105mm howitzer as its principal armament and were standard equipment for artillery battalions in U.S. armored divisions. The M4 chassis was also utilized in the M10 and M10A tank destroyers, essentially a gun motor carriage mounting a 3-inch gun, as well as the more satisfactory M36 series mounting a 90mm gun.