Mobile American Howitzer

The American army had a relatively small amount of armoured vehicles on hand at the start of WWII. Just over two years of neutrality proved enough to fully prepare for war. The American army had an impressive amount of light and medium tanks by December of 1941. They had so many tanks that they were also able to supply the British and offer significant help to the USSR. By the time the United States entered WWII they also had their first SPGs. At first these were built on halftrack chassis, but half a year later fully tracked vehicles became available. One of them was the HMC M7, better known under the British designation Priest. This was the most common self propelled howitzer of the war and it remained in service for decades after.

Experiments on a medium chassis

The Americans first worked on SPGs during the First World War. That work did not progress past experiments and work stalled after the war ended. An attempt was made to build an SPG on the chassis of the Light Tank T1, but only one prototype was built. Further work focused on vehicles equipped with the 75 mm Pack Howitzer designed for direct fire roles. 

It became obvious towards the summer of 1941 that another self propelled howitzer on a larger chassis was needed. The light chassis could not mount the 105 mm M2A1 howitzer. This system had a long history of development. The first models appeared in the late 1920s, but mass production only began in 1941. The wait was worth it, as the M2A1 was one of the best guns in its class. Suffice it to say, this weapon is still in service. The idea of mechanization was an obvious next step. This gave the weapon better mobility and significantly decreased deployment time. These guns could quickl change positions to support advancing forces.

The M2A1 howitzer was one of the main types of American artillery during the war.

The first attempt to mechanize the M2A1 took place in June of 1941. Requirements for the Howitzer Motor Carriage T9 called for using the chassis of the Medium Tank M3 that was just put into production. The speed, armour, and cruising range were to be the same as the M3. The height was limited to 2032 mm. The maximum gun elevation was to be 20 degrees and gun depression at -10 degrees. These kinds of parameters suggest that the weapon would be used for direct fire missions. Unfortunately, nearly no data remains on the HMC T9, but it was generally similar to the GMC T24 that appeared later. The project was abandoned quickly, as the HMC T9 had little promise. Instead, the HMC T19 on the M3 halftrack chassis was developed very quickly. Mass production of the HMC T19 began in January of 1942 with 324 vehicles built in total.

HMC T32 on trials, February 1942.

The HMC T19 was a necessary compromise. The Ordnance Department understood perfectly that this vehicle was just a bandaid solution. For instance, it carried only 8 rounds of ammunition on board, plus the M3 chassis was too weak for such a powerful gun. Head of the Armored Forces Major General Jacob Devers authorized the development of a new SPG on the Medium Tank M3 chassis in October of 1941. This vehicle was indexed HMC T32. It was designed with mistakes of the HMC T9 and GMC T24 in mind.

The HMC T32 was not without fault, but it still proved preferable to the HMC T19 halftrack.

Baldwin Locomotive built two prototypes that were conceptually different from the T9/T24. Since the requirements called for a low silhouette, the gun was sunk into the casemate as far as possible. The vehicle was still very tall, 2440 mm. However, the maximum gun elevation increased to 35 degrees. The traverse range to the right increased to 23 degrees since the gun was shifted to the right slightly. The ammunition capacity was quite decent: 44 rounds. The HMC T32 chassis was also used to design the GMC T40 tank destroyer which was even briefly standardized as the GMC M9.

The second HMC T32 prototype modified to account for experience gained in trials.

The first prototype was sent to Fort Knox in February of 1942, shortly after it was tested at the Aberdeen Proving Grounds. A large list of changes was composed as a result of these trials. The tankers' desires sometimes conflicted with reality, for instance they wanted the M2A1's full elevation (66 degrees) and a low silhouette at the same time. This was physically impossible. All that could be done was to increase the traverse to the right to 30 degrees. The silhouette of the HMC T32 changed dramatically. The initial vehicle had a casemate similar to the hull of the Medium Tank M3, but in the final configuration it was greatly enlarged. The fighting compartment became larger and could now fit not just the full 7 man crew but also 57 rounds of ammunition.

The ammunition was also rearranged. Previously all rounds were stored in one large rack, now most of it was kept in bins under the fighting compartment floor. The thickness of the casemate armour was reduced to 12.7 mm, but the vehicle's mission didn't require any more. Another significant change was the introduction of the M2HB .50 cal machine gun in the front right corner of the fighting compartment. All of these changes were introduced into the second HMC T32 prototype that remained at the Aberdeen Proving Grounds. Despite all drawbacks, it was clear that the HMC T32 is greatly superior to the HMC T19. For this reason, the decision to put this vehicle into production was made by March of 1942. By April it was standardized as the HMC M7.

A tricky platform

The need to put the HMC T32 into production as fast as possible was understandable. Unlike tanks, the Americans were having a tough time with SPGs. The light self propelled howitzer program failed completely and the vehicles it produced had quite questionable characteristics. Halftrack chassis had their own problems: low ammunition capacity, tall silhouette, cramped fighting compartment. The HMC T32 gave the Americans a fighting vehicle with good ammunition capacity and characteristics that allowed support with indirect fire.

The first production HMC M7, Aberdeen Proving Grounds, April 1942.

The American Locomotive Company (ALCo) was selected to produce these vehicles. This company had been building the Medium Tank M3A1 since January of 1942. The chassis of the tank and SPG were identical. ALCo's M3 tanks had a cast upper hull, but this was a temporary deviation. By the spring of 1942 it was clear that the M3A1's days are numbered and the upcoming Medium Tank M4 was going to have a fully welded hull, so ALCo was going to have to master welding anyway. The casemates of the first HMC M7 vehicles that arrived at the Aberdeen Proving Grounds in early April of 1942 were already welded. 

Unlike the experimental T32, the production M7 had a welded casemate, although the lower hull "tub" was still built with rivets.

Like the HMC T32, the M7 carried 57 rounds of ammunition. The casemate was built in a similar fashion as well, but significant changes were introduced from the start. The AA machine gun ring was now different and armoured protection was introduced along its perimeter. Racks for grousers were added to the front of the casemate and bins for personal belongings were added to the rear. The engine deck of the first M7s also changed.

Demonstration of the Browning M2HB machine gun.

The British Military Mission kept a close eye on the HMC M7 from the very start. The British had no equivalent vehicle and their SPG program was also looking quite sad. To make up for it, they ordered an impressive volume of these new vehicles: 2500 HMC M7 in 1942 and 3000 in 1943. Much like the requirements for M3 tanks these demands had little to do with reality, but in the end it was the British who used these weapons first. Later the British began using an analogous Canadian design, the Sexton.

A removable roof was tested on the fourth production vehicle. Take note of the engine deck, only the first 100 M7s built had an engine deck like this.

According to contract T-3529 ALCo built 599 vehicles with registration numbers U.S.A. W-3034235-3034833. Visibly noticeable changes were introduced from practically the very beginning of mass production. The engine deck changed after the first 100 vehicles. Air intakes protected by railings were added. These changes were introduced as a result of trials of the first two production vehicles, presumably trials showed that the engine overheated. The first list of changes was ready by May 5th, 1942, but improvements were introduced gradually. The engine deck was changed first, but that was far from the only request.

The first major change: an engine deck with additional air intakes.

Additional changes to the fighting compartment were also requested. 57 rounds of ammunition was not enough and a proposal was made to increase that number to 69. 7 additional rounds were stowed along the left side and 5 along the right. This made fitting seats complicated. The issue was solved simply by removing 4 of them. This change was not made right away, but sometime in the summer of 1942. The toolbox lids were also changed for an unknown reason, but early lids can still be seen on later vehicles.

A one piece transmission cover was introduced in the summer of 1942

The chassis of the HMC M7 changed gradually as well. As mentioned above, the chassis came from the Medium Tank M3, but it replacement in the form of the M4 was coming. The HMC M7 began to transform as a result. The transmission cover was the first sign of change. Initially it was made from three pieces bolted together. A single piece cast cover was introduced in the summer of 1942. This cover better resisted gunfire, but the old cover was still installed until the backlog was expended.

Ammunition racks changed, increasing the ammunition capacity to 69 rounds. A portion of the crew's seats were removed to make room.

The last vehicles to satisfy contract T-3529 were delivered in September of 1942, after which the second contract, T-3882, came into effect. ALCo was to deliver 2214 SPGs with registration numbers U.S.A. W- 4037519-4039732. Medium tanks M4 and M4A2 ordered by contract T-1480 also entered production in September. It was around this time that the HMC M7 received bogeys from the M4, even though the hull tub was still the riveted M3/M3A1 style. The M7 transitioned to more and more elements taken from the M4 rather than the M3. ALCo unified its chassis by early 1943. The three piece transmission cover made a comeback, although it was later replaced again. New headlight guards were added during production.

The process of unification with the Medium Tank M4 began in the fall of 1942. The M4's suspension was used and the three piece transmission cover returned.

The casemate was also undergoing an interesting transformation. One of the causes was experience in using the HMC M7 at the Battle of El Alamein, although there were others. Recall that the HMC M7's fighting compartment was completely open to the elements, and so the crew suffered from precipitation. Experiments on a removable tarp roof began in the spring of 1942. Two wire supports for the tarp were installed. If the tarp was removed, they could be carried on the side of the casemate. This change was put into production towards the end of 1942.

Guards for the headlights and a new AA machine gun turret were introduced around this time.

A bigger issue discovered during battle was that the ammunition racks were not protected from flanking fire. The rounds stuck out above the sides of the casemates, inviting enemy bullets. An order was given to urgently resolve this. Special folding plates were added to the sides of vehicles that would protect the ammunition. Additional plates were welded on to existing vehicles. Another issue was the poorly placed machine gun turret. By early 1943 it was made taller, making it easier to work in. All of these changes slowly increased the mass of the vehicle, but it was still less than its base chassis. The combat mass hit 23 tons by the start of 1943, so the power to weight ratio was still higher than for the Medium Tanks M3 or M4. 

A tarp was added in late 1942.

Contract T-3882 was satisfied in August 1943, and ALCo focused on Medium Tank M4 production. It seemed that the HMC T76 on the Light Tank T24 chassis would replace it. This vehicle was more mobile, had a more compact gun, but maintained the same ammunition capacity. Unfortunately, development dragged on, and these SPGs were needed urgently. Production of the HMC M7 had to resume. T-3882 was altered to call for 500 more vehicles with registration numbers U.S.A 4039733-4040232.

July 1943 production vehicle with late model changes like folding side panels. The suspension bogeys are the M3 type due to a backlog of parts.

1944 production HMC M7. This vehicle was very different from earlier ones and the chassis here is all taken from the Medium Tank M4.

Augmentation of contract T-3882 wasn't the only action taken to renew HMC M7 production. There was a great need in these vehicles, not just for the American army but also for Lend Lease shipments. It was necessary to increase the number of manufacturers. The chassis of the Medium Tank M4 was no longer optimal at that point. The Medium Tank M4A3 with its more powerful and simpler Ford GAA engine was a better choice. The decision was made to put the HMC M7 into production on a new chassis.

500 HMC M7 SPGs were built in 1944. 176 more were built in 1945 by Federal Machine & Welder Company.

Production of the HMC M7B1 began in March of 1944. These vehicles were built using the chassis of the Medium Tank M4A3.

Differences of the HMC M7B1 can be seen from above or from behind.

The Battle of El Alamein was the Priest's trial by fire.

The British received over 800 vehicles of this type in total.

HMC M7 during the fighting in Sicily.

Some of the vehicles had additional armour on the sides to protect the ammunition.

The HMC M7 was very common by the fighting in Normandy.

HMC M7B1 from the 11th Armored Division, 1945. The trailers came with the M7 and contain additional ammunition.

These vehicles were rare in the Pacific.

HMC M7B2, a special Korean variant.

Increasing the height of the gun mount allowed the gun elevation to be increased to 65 degrees.

Semovente DA 105/34, an Italian modification of the HMC M7.