Monday 8 October 2018

Overseas Doorknocker

At the time the United States joined WWII the main anti-tank gun of the American army was the 37 mm M3 gun. As experience in North Africa showed, this gun could not successfully combat modern tanks. While the 37 mm gun was replaced by more powerful weapons in Europe, the US Army and Marines used these guns in the Pacific theatre until the end of the war.

The United States was far behind European nations in anti-tank weapon development. Leading European armies were beginning to adopt small caliber AT guns by the start of the 1930s, while the Americans only began thinking about adopting them in the middle of the decade. To start, the Americans decided to research foreign experience. One French Canon 25 mm S.A. Mle 1934 gun was purchased in 1935. This gun did not leave a good impression on the Americans: despite its excellent penetration, it was considered too heavy (475 kg, more than the Swedish or German 37 mm gun).

The first battles of the Spanish Civil War showed how important anti-tank artillery was. Successful use of armour did not evade the attention of the Americans either, and in January of 1937 the US Army command decided to develop a 37 mm gun. The gun was supposed to have a low mass (no more than 1000 pounds or 454 kg) and its shell had to penetrate 20 mm of armour at 800-1000 meters. These parameters were based on the thickness of armour of modern tanks and the maximum range of their machineguns, which was 800 meters. In Spain, the Americans took note of the German 3.7 cm Pak anti-tank gun, and bought a sample later. This fact led some historians to insist that the American gun is a copy of the German weapon, but in reality it is an independent design.

Design and improvement

Three branches of the military (cavalry, infantry, and artillery) met at the Aberdeen Proving Grounds in May of 1937 to discuss the development of anti-tank weapons. This discussion defined the requirements for the prospective gun. Having examined the 3.7 cm Pak, the participants decided to borrow the design of the sight, somewhat reminiscent of a submarine periscope, which allowed the gunner to aim while still being protected by the gun shield. The artillery branch insisted on a layout similar to field guns, with two gunners to the left and right of the breech, one of which would aim the gun vertically and the other horizontally. The infantry protested, defending the layout where only one gunner was necessary. The most important result of this meeting was to give the development of the new gun to the infantry command. This made it an infantry weapon, and not an artillery one. The Bennet laboratory, a part of the Watervliet arsenal, took up the development.

The design was finished quickly. On September 9th, 1937, the blueprints for the future weapon were approved, and trials of a prototype began at the Aberdeen Proving Grounds on in March of 1938. The gun and carriage received the indexes T3 and T1 respectively.

T3 gun on the T1 carriage.

The first trials showed the drawbacks of the gun, or rather the carriage: poor stability when aiming away from the main axis and slow speed of horizontal aiming. The next modification, indexed T1E1, attempted to resolve these problems. It received a two-speed horizontal aiming mechanism (one speed for rough and one speed for precise aiming), but the stability was not improved. In addition, the penetration was insufficient, and the gun jammed often. The customer rejected this gun. The designers rolled up their sleeves and put out three more prototypes with different breech designs: the T7 had a horizontal semiautomatic sliding breech, the T8 had a Nordenfelt eccentric screw breech, and the T10 had a classic vertical sliding breech. All three had the new T5 carriage. The T10 gun showed the best results, and on December 15th 1938 it was standardized as the 37 mm Gun M3 and recommended for mass production. The T5 carriage was named M4 Gun Carriage.


The M3 gun had a monobloc 2.1 meter (56.6 calibers) barrel with 12 right hand rifling grooves. The breech was a vertical sliding breech, hand operated. The firing mechanism was hand fired, spring loaded, and had to be cocked manually after loading. Overall, the design of the breech and firing mechanism was simple and reliable. Despite a lack of automatic mechanism, a well trained crew could make 25 shots per minute.

M3 gun on the M4 carriage.

The recoil system combined springs and hydraulics. The carriage had a split trail with a hitch on the right spade. The wheels had pneumatic tires, the axle was unsprung. Extra trails could be flipped down under the wheels to give the gun additional stability when firing. On January 29th, 1942, the M4A1 carriage was standardized, which gave additional aiming speed. To achieve this, the aiming mechanism could be disengaged, and the gunner could turn the barrel by hand using a special grip on top and to the left of the breech. The gun was aimed using the M6 telescopic sight. A 12.7 mm shield protected the crew.

The crew of the M3 consisted of five men: the commander, gunner, loader, and two ammunition carriers.

The crew could move the gun by hand over short distances/

The gun fired fixed ammunition. There were five types of ammunition: M74 AP and M51 APC were shot without explosive filler. They weighed 870 and 940 grams respectively, and the mass of the whole round was 1.51 and 1.58 kg. The muzzle velocity was 870 m/s. The penetration for the M51 round at 30 degrees was 53 mm at 500 yards (457 m) and 46 at 1000 yards (914 m). The M63 and Mk.II HE shells weighed 730 and 560 grams respectively, and were filled with 39 and 27 grams of explosives. The whole round weighed 1.42 and 1.23 kg. The muzzle velocity of the M63 was 782 m/s. The effectiveness was about the same as that of a hand grenade, which meant the ammunition could be used to destroy MG nests and unarmoured vehicles. The fifth type of ammunition was the M2 canister shot, armed with 126 round steel pellets.

The training tracer TP M51 ammunition was used for training, as well as the M13 and T5 dummies (matching the shape of the M51 and M63 rounds), as well as the M2 blank round weighing 930 grams.

On March 5th, 1942, the M3A1 gun was standardized, which had a threading on the barrel for the attachment of a Solothurn muzzle brake. However, practice showed that it led to preemptive detonation of HE ammunition, and the muzzle brake fell out of use.

In October of 1942, airborne command proposed a carriage design with detachable trails so it could be dropped from the air. Such a carriage was built and tested, but rejected in June of 1943 due to a "lack of need". The Marine Corps also tried to "tune" the M3 for their needs. The Marines were dissatisfied with the small gun shield, and in May of 1944 a larger shield with irregular edges was proposed. The edges made hiding the gun easier. A few prototypes were produced and built in late 1944, but that was the end of it. By this time 37 mm guns were useless even in the Pacific.

A "customized" M3 equipped with an enlarged gun shield with an irregular outline. Iwo Jima, February 21st, 1945.

Production of the M3 began in 1940. The guns were built at the Watervliet arsenal, and the carriages at Rock Island Arsenal. 18,072 guns were built (340 in 1940, 2252 in 1941, 11,812 in 1942, and 4298 in 1943).


The 37 mm anti-tank guns were included in the TO&E of American infantry divisions in 1941. Each infantry battalion received a platoon with three M3 guns, and each infantry regiment received a nine gun company. 3/4 ton Dodge WC-51 trucks were used to tow them, but lighter jeeps were also successfully used. This had an impact on the altered TO&E adopted in April of 1942: now platoons had their guns towed by jeeps, but the companies retained their Dodges. At the same time, the number of guns increased to four per platoon and 12 per company. Now each regiment contained 24 M3 guns. Each of the four artillery battalions of the artillery regiment received a six-gun platoon. They were supposed to be used to protect artillery positions from tank attacks. Engineer battalions had anti-tank companies with nine M3 guns towed by M2 halftracks. Finally, six guns were kept at the division HQ. Each US infantry division totalled 111 37 mm guns. In addition, at the start of the war the divisions had anti-tank battalions (two companies of 12 M3 guns and one with 12 75 mm M1897A4 guns), but these were quickly taken out and turned into independent tank destroyer battalions.

The 3/4 ton truck was the standard tractor for the gun.

The 37 mm gun could also be towed with a jeep (in this case an American Bantam).

According to the TO&E issued in October 1942, each airborne division received an anti-tank battalion with two companies of 12 M3 guns each and a platoon with four guns in the artillery battalion. The glider regiment had a company with 8 M3s. A division that had one glider regiment would have 36 M3 guns, and a division with two of them would have 44.

M3 gun crew on maneuvers, Tennesee, June 1941.

As of March of 1942, tank divisions had the following distribution of 37 mm guns: 37 in the motorized regiment (four in each motorized infantry company and one in the HQ), 27 in the engineer battalion, and 4 in command units. In total, the division had 68 M3 guns, but in 1943, when tank divisions were refactored from a regimental to a battalion structure, the 37 mm guns vanished from their ranks.

M3 guns served in Alaska, although they didn't encounter enemy tanks here.

The Marine Corps had the same number of M3 guns as the army in their battalion-regiment units: 4 guns per battalion (in the weapons company) plus 12 at the regimental level (in the anti-tank company of the weapons battalion). In addition, the division had a three-battery battalion of M6 SPGs (M3 guns mounted on a Dodge truck), 18 in total. 37 mm guns were taken out of battalions in April of 1943, and the SPG battalion was moved over to towed M3 guns, more suitable for fighting in the jungle. In May of the next year they were removed due to a lack of necessity. The odds of meeting tanks in the Pacific were slim compared to Europe or North Africa. A Marine division had 90 M3 guns in 1942 (18 of them self propelled), but only 36 in 1944.

Use in battle

The combat debut of the M3 happened in December of 1941 during the defense of the Philippines. Information on their use in this battle is sparse, but one episode demands closer attention. On February 8th, 1942, a squad of 20 airmen left without a ride and fighting as infantry landed at Agloloma Bay, on the south of Bataan, in the rear of the Japanese forces. Thanks to an attack by the American landing force, the Japanese were beaten back from favourable positions and lost 600 men. The landing was entirely improvised. The troops landed on two whale boats towed by 11 meter motorboats. The motorboats were equipped with improvised armour made of steel plates and each had a 37 mm gun installed. The gunboats offered fire support for the landing force with a 10 minute artillery barrage. This landing was the first American amphibious operation in WWII.

Fillipino scouts train with an M3 gun, 1941.

The US Marines used M3 guns starting with October of 1942 in the battles for Guadalcanal. Defending their positions at the Matanikau river, the Marines destroyed the Japanese 1st Independent Tank Company armed with Chi-Ha medium tanks. The 37 mm guns also proved effective at defending against infantry attacks. Canister shot mowed down the attackers.

Jungle warfare was not an easy task. A crew from the 7th Infantry Division pulls their M3 gun behind them. Kwajalein, February 1944.

An M3 gun supports Marines at Saipan, June 1944.

While the Japanese possessed only a handful of thin-skinned tanks, the Americans encountered a more serious opponent in November of 1942, after landing in North Africa. The Germans had PzIII and PzIV tanks, and the M3 could do little against their front armour. By then, both the allied British and the Germans had more powerful anti-tank guns (57 mm and 50 mm respectively), while the Americans had to settle for 37 mm cannons. The result was predictable: at the battle for Kasserine Pass 67 M3 guns were destroyed or captured. The commission that studied the result of fighting in Tunisia was ruthless in its conclusions, given on March 5th, 1943.
"These guns were useless as anti-tank weapons and we insist that they are disposed of. They cannot penetrate the front armour or turret of German medium tanks, their rounds bounce off their armour like peas, while German tanks crush our positions..."
To be fair, there was also another opinion. Brigadier Thomas Louis wrote after observing trials against a captured German PzIII tank:
"The 37 mm gun, used with caution, meaning firing at the flanks and avoiding fire from excessive distances, is an effective anti-tank measure." 
The M3's size made it easy to conceal.

New TO&E issued in May of 1943 replaced the M3s with 57 mm M1 guns (copies of British 6-pounders). However, these weapons only appeared in large amounts in early 1944. The campaign in Sicily was fought with 37 mm guns, which were still effective against Italian tanks and captured French Renault R35s. When German tanks appeared on the battlefield, the infantry's only hope was their tank destroyer battalions, armed with M10 SPGs.

By the landings in Normandy, the 57 mm M1 was already the infantry's main anti-tank weapon. In the Pacific theater the guns remained in use until the end of the war. Their small size came in handy when fighting in the jungle.

An M3 gun crew of the 7th Cavalry Regiment at a checkpoint near Santo-Tomas (Luzon, Philippines). March 25th, 1945.

The only notable receiver of this gun through the Lend Lease program was China (1669 units). The second largest supply was to Chile, who received 198 guns. Small batches ended up in other Latin American countries: Paraguay (12), El Salvador (9), Columbia and Bolivia (4 each). Cuba received one single M3. The Free French received 130 of these guns, Great Britain got 78, the USSR 63, and Canada 3.

37 mm M3A1 with a Solothurn muzzle brake and the M5 3" gun. The difference can be seen with the naked eye.

The American 37 mm gun arrived on the scene much later than its analogues, especially the German 3.7 cm Pak "doorknocker". As a result, it was obsolete by the time it entered service, which guaranteed its relatively short and unsuccessful career.


  1. I think you want to replace the term "squadron" with "battery" in this otherwise fine article.

  2. It may have been rather dated already by the time of introduction but it certainly beat some of the OTHER ideas the Army looked into...

    1. Not all of those were a waste, investments in the rock paid off, and the military uses it (or something similar) to this day.

  3. That gun shield seems a little thick. Other sources put it at half that thickness. I don't know first hand, however.