The 105mm Howitzer Motor Carriage M7

By James Lankford

At the outbreak of World War II, the U.S. Army was ill-prepared for the mechanized, maneuver warfare employed by the German Wehrmacht. This was especially true of the Army’s nascent Armored Force. In the summer of 1941, the Armored Force was struggling to develop doctrine and equipment for its armored divisions. The M7 105mm Howitzer Motor Carriage was an important result of that effort. 

On 1 August 1941, Major General Jacob L. Devers became Chief of the Armored Force. An artilleryman, Devers had a reputation for swiftly solving problems. Among those he faced was an urgent need for an armored, self-propelled 105mm howitzer to replace the towed artillery currently in use. Unlike towed artillery which operated in the rear, Devers wanted artillery capable of providing close fire support to the leading elements of his armored divisions. His Chief of Artillery, Brigadier General Edward H. Brooks, suggested basing the new artillery on the M3 medium tank chassis. The M3 was already in production and sufficient chassis were available for the proposed armored howitzer motor carriage.

On 1 October Devers recommended that two prototypes be built using the M3 tank chassis armed with the M2A1 105mm Howitzer. The prototypes, designated T32, were produced by Baldwin Locomotive Works. During the initial testing phase, some rear armor was removed to provide more room for servicing the howitzer. A single prototype was sent to Fort Knox, Kentucky, for evaluation by the Armored Force Board. The prototype arrived at Fort Knox on 5 February 1942 and “…and before the end of four days, it had been shipped away for modifications and eventual production.” 

The requested modifications included an increase in the height of the frontal armor for better crew protection, the side armor was lowered to facilitate access to the fighting compartment, and the howitzer traverse was increased to forty-five degrees. The addition of an M2 .50 caliber dual purpose machine gun was also requested. These modifications were made to the second prototype at Aberdeen Proving Ground, Maryland, before it was sent to the American Locomotive Works for production. To ensure work progressed as fast as possible, Devers assigned Brooks to oversee the project.

While he waited for his new artillery, Devers obtained the production of 314 half-track-based T19 105mm Howitzer Motor Carriages for training purposes. They became available in January 1942 and were issued to the 1st and 2d Armored Divisions as well as some infantry regiment cannon companies.

In March, the first M7s rolled off the assembly line. Following another evaluation by the Armored Force Board and the Ordnance Department, the M7 was accepted for service. The M7 was standardized for use in April 1942, roughly two months after the Armored Force Board approved the T32 prototype. Armored divisions were issued M7s as they became available.

The armored division reorganization of 1 March created three armored field artillery battalions containing fifty-four 105mm howitzer motor carriages. Each battalion had eighteen howitzers with six in each of its three firing batteries.  

In August 1942, the Armored Force published FM 17-60, Armored Division Artillery. The document transformed the role of artillery. Armored division artillery would provide close fire support to the tanks and infantry. Firing batteries were routinely assigned to the division’s leading elements, aka “point guards.” The artillery would fire preparations prior to attacks, then follow through with the attack and provide close support as needed. The artillery was also expected to fight hostile tanks when necessary and provide defensive fires. The M7 made all this possible.

The M7 weighed 50,634 pounds. Its maximum speed was twenty-five miles per hour and it could climb a grade of sixty percent. It was capable of crossing trenches up to seven feet, six inches wide and clear vertical obstacles of twenty-four inches. Fording depth was forty-eight inches and the turning diameter was sixty-two feet. Fuel capacity was 179 gallons with a range of 85 to 125 miles depending on conditions. The ground pressure was 10.4 pounds per square inch, twenty-eight percent lower than the M3 tank. The overall exterior dimensions were similar to the M3. The length was 19 feet, 9 inches and the width was approximately 9 feet, 5 inches. The height was 9 feet, 8 inches at the top of the machine gun.

The M7 was powered by the 340 horsepower, air-cooled Continental R-975-Cl gasoline engine used in the M3 tank. The nine-cylinder engine was adapted from a Curtis Wright aircraft engine. The tank version was produced by Continental Motors under a licensing agreement. Power was transferred through a synchronized transmission with five forward gears and one reverse.   

The fighting compartment was open-topped; a canvas tarp supported by curved rods was supplied to protect the crew from the elements. The crew entered via steel ladder rungs welded to the sides.

 The frontal lower armor ranged from 2 to 4.5 inches in thickness. The upper armor was .5 inch thick. The lower side armor was 1.5 inches and the upper portion was .5 inch. The lower rear armor was 1 inch and the upper was .5 inch. The side armor offered protection from small arms fire and shell fragments.

The main gun was the 105mm Howitzer M2Al on the M4 Mount. The rate of fire was four rounds per minute. The M7 fired the same ammunition as the towed M2A1. Maximum range was 12,500 yards (7.1 miles). The howitzer could be elevated from -5 to 35 degrees. The elevation was significantly less than the sixty-five-degree elevation of the M2A1 towed version because of the vehicle floor. High-angle fire required placing the entire vehicle on an incline, which was not always possible.

The 105mm howitzer could be traversed thirty-five degrees to the right and fifteen degrees to the left. The need to reposition the vehicle to allow greater traverse while keeping the sights on the aiming stake was initially seen as a serious limitation. This argument was contested in an article published in the December 1943 issue of the Field Artillery Journal in which the author asserted the difficulty could be overcome with skill and teamwork: “The only answer is drill, drill, and more drill.” Relevant reports of the General Boards written shortly after the end of the war do not mention this as a problem.

In May 1942, the onboard ammunition storage for the howitzer was increased from fifty-seven to sixty-nine rounds. Two of the crew seats were removed to make room for the additional ammunition racks. Extra ammunition was carried in trailers towed by the M7s and half-tracks of each battery. Each trailer carried forty-two complete rounds. Replenishment was provided by trucks from the battalion service battery.

A single .50 caliber M2 heavy barreled machine gun was mounted on a 360-degree rotating ring to the right of the howitzer. The standard ammunition load for the M2 was 300 rounds. The rounded machine-gun position was the highest part of the armor. It was later raised and widened to add a seat for the machine gunner and provide more protection.   

The M7 had a seven-man crew consisting of a driver, chief of section, gunner, and four cannoneers. Personal weapons for the crew consisted of three .45 caliber submachine guns. The standard ammunition load for each weapon was 540 rounds. Direct vision for the driver was through a removable windshield. Indirect vision was through a “protectoscope.” A panoramic telescope and an elbow telescope provided additional visibility. 

The M3 and M4 tanks shared the same chassis. As a result, the M7 chassis evolved along lines similar to those of the M4 chassis. The early three-piece Iowa transmission was replaced by the Caterpillar transmission with the one-piece cover used on the M4 during the summer of 1942. However, demand for the Caterpillar transmission increased as M4 tank production accelerated. This necessitated the use of both transmission types on the M7. Production M7s were upgraded with the stronger bogies used on the M4. Stowage capacity was increased with the addition of external boxes. In the fall of 1943 armored, folding panels were added to the sides and rear to provide extra crew protection. Combat experience also revealed the need for additional armor to protect the ammunition ready racks.     

In August 1943 production ceased when the initial order for 2,814 M7s was fulfilled. A major reorganization of the Army Ground Forces in September required more M7s. Production resumed the following March. A total of 3,490 M7s were produced during the war.

The need for additional M7s led to the production of the M7B1. Based on the M4A3 Medium Tank chassis, the M7B1 used the tank’s V8 Ford GAA 500 horsepower engine. The new engine was more powerful and more easily maintained than the Continental radial. Pressed Steel Car Company was awarded the contract and delivered the first M7B1s in March 1944. A total of 826 units were produced with production ending in February 1945. Generally speaking, there were few differences between the M7 and M7B1 with the following exceptions. Top speed was increased to twenty-six miles per hour, the overall length was roughly seven inches longer, and the fuel capacity was reduced to 168 gallons. However, the cruising range was slightly better. Fording depth was reduced from forty-eight to thirty-six inches.

On 20 June 1942, Field Marshal Erwin Rommel’s Afrika Korps captured the key port city of Tobruk from the British Eighth Army. The German success threatened British control over Egypt and the vital Suez Canal. In desperation, Prime Minister Winston Churchill asked President Franklin D. Roosevelt for assistance. The next day Roosevelt authorized an emergency Lend-Lease shipment of 300 M4 Sherman tanks and 100 M7s to Egypt. The tanks and M7s were drawn from the 1st and 2d Armored Divisions and other sources. They arrived at Suez in September 1942.

The M7’s rounded, open-topped .50 caliber machine-gun compartment reminded British troops of a priest’s pulpit. As a result, they named it the “Priest.” On 24 October 1942, the M7s were first used in the Battle of El Alamein as part of the Royal Armored Corps. Afterward, the British concluded the M7 was “a great success.”

The British received a total of 832 M7s under Lend-Lease. Over time, the British replaced the M7s with British and Canadian-built self-propelled artillery with their standard howitzer, the 25-Pounder. As a result, many surplus M7s were converted to armored personnel carriers by removing the howitzer and ammunition racks. These variants were known as “Defrocked Priests” or Kangaroos. Another British conversion carried the 9.75-inch Chemical Mortar which was intended for use against fortifications. Only three were produced. They were offered to the U.S. Seventh Army as it approached the Siegfried Line in March 1945, but they were found unsuitable for combat.  

The French lost most of their military equipment to the Germans early in the war. What little they had in North Africa under the Vichy Government was worn and outdated. After the Allied landings in North Africa, the British and Americans rearmed the French under Lend-Lease. During the war, the French received 283 M7s. 

The U.S. Army’s first combat use of the M7 was in North Africa during Operation TORCH. The 1st Armored Division’s artillery battalions were equipped with one battery of M7s and two batteries of the T19, the half-track-based 105mm Howitzer. The 2d Armored Division also had some M7s, but the total number is unclear. In January 1943 three armored field artillery battalions equipped with M7s were used to equip the 5th Armored Field Artillery Group with the mission of supporting the armored divisions. One of the group’s battalions fired its first rounds against the enemy on 20 March. In May a battery of M7s from the same battalion engaged enemy tanks with direct fire destroying one while forcing the others to withdraw.

During the war, the Army created seventy-three armored field artillery battalions equipped with M7s. Of these, forty-eight went to armored divisions and twenty-five went to non-divisional battalions created to provide additional fire support for the armored divisions. By 1945, eighteen non-divisional battalions were in Europe and three in the Pacific. The remaining four remained stateside for training purposes. The battalions sent to Europe primarily supported armored divisions although they were also assigned to support infantry divisions. Several regimental cannon companies also received M7s.

Late in the war, the Marines began replacing the half-track mounted M3 75mm guns used in their regimental cannon companies with M7s. The 1st and 6th Marine Divisions fielded four M7s in each of their cannon companies during the Battle of Okinawa.

Despite its limitations, an official postwar assessment concluded the M7 l05mm Howitzer Motor Carriage was very successful in fulfilling its mission of direct support to the armored division. It provided protection for the crew while maintaining tactical mobility. The 105mm howitzer was considered the best weapon for direct support of armored forces. The M7 was used successfully in all types of operations including assault landings, supporting attacks, exploitation, pursuit, defense, fighting withdrawals, and river crossings, and it was a major factor in the success of the armored divisions it supported.

The M37 105mm Howitzer Motor Carriage began production in October 1945 as a replacement for the M7. It was based on the M24 Chaffee light tank chassis and armed with a modified M4 105mm howitzer used on a variant of the M4 Sherman. It was smaller, slightly faster, and more maneuverable than the M7. The M37 was first used in combat during the Korean War. Only 316 were produced.

Early in the Korean War, the limited thirty-five-degree elevation of the M7’s howitzer proved problematic. Greater elevation was required to hit targets in the high mountains of Korea. M7B1s were fitted with a modified M7J1 gun mount was used to increase the elevation to sixty-five degrees. This was accomplished by raising the gun higher from the compartment floor. The armor and machine-gun pulpit were also raised. A total of 127 M7B/2s were produced.

As the Cold War grew colder, excess M7s, M7B1s, and M7B2s were sold to NATO and other countries for their armed forces. The total number of countries that obtained M7s is unclear.