Design and Development: The M3

The US Army did not possess a medium tank in the 1930s, although several developmental designs existed on paper, and in some cases, prototypes were built through the 1920s. These exhibited British influence and bore small-caliber, lowvelocity guns. In May 1936 the Army’s Ordnance Committee directed the development of a new tank based on Infantry Board requirements.2 Designated the T5, the projected medium tank was an enlarged M2 light tank, the predecessor of the M3 and M5 Stuarts. Some M2 components were used in the T5, but the new tank was to have heavier armor and armament. The T5’s armament was little different from that on its light counterpart. It was envisioned as an infantry support tank and not for tank-to-tank combat.

The T5’s chassis was not unlike the future M3’s. The front glacis was well sloped, but the superstructure’s sides and rear were vertical and further made vulnerable by numerous vision/pistol ports. The centerline driver’s position had large shuttered ports to the front and sides.

The Ordnance Department had promised M3 plans to Chrysler by November 1940, but they did not arrive until February 1941. The first hand-built pilot model was run out on April 12 and shipped to Aberdeen Proving Grounds in Maryland on May 3. The tank arsenal commenced production in July 1941 with the first production model completed on July 8. An additional order for 1,200 M3s was placed that month. In the same month that the M3 contract was let, the design of the T6 (M4) Sherman medium tank was submitted. An M4 prototype was delayed until the M3 entered production. The M4’s design was approved in April 1941 with the goal of rapidly massproducing a fast, reliable tank. The design borrowed heavily from the M3 and incorporated improvements learned from British experience in Africa. Borrowing from the M3’s design helped speed up development and made it easier to convert production lines. The first pilot M4 was turned out in September, standardized in October, and full production began in February 1942. It would replace the M3 by August 1943.



Most medium tanks of the era, including the PzKpfw III, were laid out similarly. The 360-degree revolving turret was situated just forward of center. It mounted the tank’s main gun plus a coaxial machine gun. The turret was manned by the tank commander, gunner, and loader. The driver and radio operator were in the forward portion of the hull; the latter was also the bow machine gunner. The engine was in the rear, protected by a heavy, ventilating louvered cover. The fuel tanks were usually on either side of the engine. Most tank designers strove to maintain a low profile and to incorporate as much sloped armor as possible to improve the armor’s ballistic efficiency. The M3 deviated significantly from the typical tank layout. The most noticeable feature of the 31-ton M3 was its inordinately high profile, due to its secondary gun turret and commander’s cupola. The mounting of the main gun in a right front sponson and the secondary gun mounted on a small turret also deviated much from conventional tank design. While the glacis was well sloped, much of the superstructure was only slightly angled or vertical, especially the sides and rear of the superstructure and hull.

The driver sat on the centerline, rather higher than most drivers. This gave him a better field of observation but with greater exposure. To his left were two fixed bow machine guns. After June 1942 the leftmost gun was deleted as being redundant. One or two M2 tripods were provided for dismounted use. A large driver’s hatch with an indirect vision device was set at head level. Early M3s had only a vision slit that could be used as an awkward exit hatch. Beside the driver’s left shoulder was an observation/pistol port. In the right front portion of the hull was the main gun, either an M2 or M3 75mm gun on an M1 mount. This was a large, horizontally rotating curved shield for traversing, and set in it was the smaller, elevating gun shield. The gunner sat to the gun’s left and the assistant gunner behind him. The gunner aimed using an M1 periscope with an integral M21 telescope situated in the top of the hull, but an M15 optical sight was later mounted beside the gun. A hatch was positioned over the assistant gunner’s seat. The radio operator sat behind the driver, just forward of the turret. This became the assistant gunner’s seat after the radio operator was deleted, at which time the driver took over as radio operator. This conserved manpower, but increased the driver’s workload. On both sides of the superstructure were escape hatches with inset observation/pistol ports. These ports were fitted with bulletproof glass vision blocks called “protectoscopes.”

An observation/pistol port was set in the right rear of the superstructure over the engine compartment. M3A3s and later models had the side escape hatches welded shut, and an escape hatch was provided in the belly, but this made it more vulnerable to mines. These side hatches allowed spalling – bullet and shell fragments – to enter through seams. The secondary gun turret was on a 54.5in.-diameter ring with a well-sloped front and offset to the left rear of the superstructure. The 37mm M5 or M6 gun on an M24 combination mount had a machine gun fitted to the right and the sight to the left. The commander was to the left of the 37mm and had a cupola atop the turret, mounting a machine gun in the right side and a vision port in the left. The cupola could be rotated independently of the turret. Being manual it was too slow to effectively track aircraft, its intended 12 purpose. The commander’s seat was fitted inside the turret, but the 37mm gunner’s seat was on the turret floor to the gun’s left and the assistant gunner’s to the right of the gun on the turret shield. An observation/pistol port was set in the turret’s right rear quadrant. The turret crew was separated from the fighting compartment by a circular shield extending down into the superstructure. There were ventilators aft of the assistant gunner’s hatch atop the superstructure, to the left of the driver on the roof, and on the right top of the turret. Flat equipment boxes were fitted to the glacis below the driver’s port and over his roof and two large boxes on the sides of the engine deck. There were often removed.



The five-speed transmission was in the forward portion of the hull, with the propeller shaft running beneath the floor back to the engine. The auxiliary power generator, providing electrical power to operate the turret, radio, and lights when the engine was shut down, was in the back of the fighting compartment. The engine was in the rear hull with the air-cooled radiator aft and fuel tanks containing 175gal on either side. The engine was accessed through rear twin doors on most models. A substantial amount of ammunition was carried in the M3: 46 rounds of 75mm, 178 rounds of 37mm, 9,200 rounds of .30cal for the three or four machine guns, 1,200 rounds of .45cal for the two Thompson submachine guns and six pistols, and 12 hand grenades (4xMk II fragmentation, 4xMk III offensive, 2xAN/M8 white smoke for screening, and 2xM14 thermite for destroying the guns if abandoned).



M3 production began in August 1941, and there were six US variants of the M3 tank. Five companies produced the variants, which were mostly of riveted construction. The bow was cast in three sections and riveted together, common to all variants. The 37mm turret and cupola were cast on all models. The M3 was powered by a Continental R-975- EC2 or EC1 radial aircraft engine. This was a nine-cylinder engine, burning 92-octane gasoline. The same engine powered the M3A1 and M3A2 variants. The M3 saw the largest production run and was the only one to see combat in US hands in Africa. It could be fitted with either 75mm M2 or M3 guns and 37mm M5 or M6 guns. M3 production ceased in August 1942


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