M4 Sherman

U.S. design prototype

Cutaway Sherman showing transmission and driver's seat

The U.S. Army Ordnance Department designed the M4 medium tank as a replacement for the M3 medium tank. The M3 was an up-gunned development of the M2 Medium Tank of 1939, in turn derived from the M2 light tank of 1935. The M3 was developed as a stopgap measure until a new turret mounting a 75 mm gun could be devised. While it was a big improvement when tried by the British in Africa against early German tanks, the placement of a 37 mm gun turret on top gave it a very high profile, and the unusual side-sponson mounted main gun, with limited traverse, could not be aimed across the other side of the tank. Though reluctant to adopt British weapons into their arsenal, the American designers were prepared to accept proven British ideas. British ideas, as embodied in a tank designed by the Canadian General Staff, also influenced the development of the American Sherman tank. Before long American military agencies and designers had accumulated sufficient experience to forge ahead on several points. In the field of tank armament the American 75 mm and 76 mm dual-purpose tank guns won the acknowledgement of British tank experts.^[13] Detailed design characteristics for the M4 were submitted by the Ordnance Department on 31 August 1940, but development of a prototype was delayed while the final production designs of the M3 were finished and the M3 entered full-scale production. On 18 April 1941, the U.S. Armored Force Board chose the simplest of five designs. Known as the T6, the design was a modified M3 hull and chassis, carrying a newly designed turret mounting the M3's 75 mm gun. This would later become the Sherman.^[14]

The Sherman's reliability resulted from many features developed for U.S. light tanks during the 1930s, including vertical volute spring suspension, rubber-bushed tracks, and a rear-mounted radial engine with drive sprockets in front. The goals were to produce a fast, dependable medium tank able to support infantry, provide breakthrough striking capacity, and defeat any tank then in use by the Axis nations.

The T6 prototype was completed on 2 September 1941. The upper hull of the T6 was a single large casting. It featured a single overhead hatch for the driver, and a hatch in the side of the hull. In the later M4A1 production model, this large casting was maintained, although the side hatch was eliminated and a second overhead hatch was added for the assistant driver. The modified T6 was standardized as the M4, and production began in February 1942.^[15] The cast hull models would later be re-standardized as M4A1, with the first welded hull models receiving the designation M4. In August, 1942, a variant of the M4 was put forth by the Detroit Arsenal to have angled, rather than rounded hull and turret armor. The changes were intended to improve the tank’s protection without increasing weight or degrading other technical characteristics.

Doctrine

A Sherman DD amphibious tank of 13th/18th Royal Hussars in action against German troops using crashed Horsa gliders as cover near Ranville, Normandy, 10 June 1944

As the United States approached entry into World War II, armored employment was doctrinally governed by Field Manual 100–5, Operations (published May 1941, the month following selection of the M4 tank's final design). That field manual stated:

The armored division is organized primarily to perform missions that require great mobility and firepower. It is given decisive missions. It is capable of engaging in all forms of combat, but its primary role is in offensive operations against hostile rear areas.^[16]

The M4 was, therefore, not originally intended primarily as an infantry support tank. It placed tanks in the "striking echelon" of the armored division, and placed the infantry in the "support echelon", without directing that tanks should only seek to attack other tanks, thus leaving target selection up to the field commander based on what types of units were available to him to attack. A field manual covering the use of the Sherman (FM 17–33, "The Tank Battalion, Light and Medium" of September 1942) described fighting enemy tanks when necessary as one of the many roles of the Sherman, but devoted only one page of text and four diagrams to tank-versus-tank action, out of 142 pages.^[17] This early armored doctrine was heavily influenced by the sweeping early war successes of German blitzkrieg tactics. By the time M4s reached combat in significant numbers, battlefield demands for infantry support and tank versus tank action far outnumbered the occasional opportunities of rear-echelon exploitation.^{[citation needed]}

United States doctrine held that the most critical anti-tank work (stopping massed enemy tank attacks) was primarily to be done by towed and self-propelled anti-tank guns, both of which were referred to as "tank destroyers", with friendly tanks being used in support if possible.^[18] Speed was essential in order to bring the tank destroyers from the rear to destroy incoming tanks. This doctrine was rarely followed in combat, as it was found to be impractical. Commanders were reluctant to leave tank destroyers in reserve; if they were, it was also easier for an opposing armored force to achieve a breakthrough against an American tank battalion, which would not have all of its anti-tank weapons at the front during the beginning of any attack.^[19]