The United States built almost 19,000 tank destroyers during WWII. There was a great variety of them: tracked, half-tracked, and wheeled vehicles. The only class of vehicle that the Americans never turned into tank destroyers were heavy tanks. The American concept of a tank destroyer, a tank with lighter armour but a more powerful gun in an open turret, bred many other solutions that were later rejected. Let us recall some of these dead ends.
It is often thought that the Americans began work on tank destroyers after WWII began, but engineers started thinking about destroying enemy tanks in the mid 1930s. At the time, typical armament for American tanks was a pair of machineguns, one rifle caliber, and one large caliber (12.7 mm).
It was proposed to either use the short barrelled 37 mm infantry gun or an experimental 37 mm tank gun converted from the Browning semiautomatic AA gun equipped with a 5 round magazine. This gun was trialled on the T1 family of light tanks, but nothing came of it. As for the 37 mm infantry gun M1916, its armour piercing effect was even lower than that of the Browning M2HB that was installed in the light M2 and M1 Combat Car series tanks. Not surprisingly, the machinegun was preferred.
The American military realized that the AT capabilities of the heavy machinegun were limited. As soon as production of light tanks began, design of tank destroyers on their chassis started. The M2A1, America's first mass production light tank of the 1930s, served as a lab rat. Unlike the double turreted M2A2 and M2A3, their predecessor had one two-man turret with enough space to install a cannon.
Work on a tank destroyer on the M2A1 chassis was initiated by the Infantry Department on October 9th, 1936. The tank destroyer, classified as a tank support vehicle, received a 47 mm semi-automatic Browning cannon. This was effectively the same 37 mm semi-automatic cannon, but with a larger caliber. It was first trialled on the T2 Medium Tank. Like its smaller version, it was loaded with 5-round magazines.
The tank destroyer was built in July of 1937 in Fort Benning, Georgia. "Built" is a strong word, as it was more of a conversion. The stock turret was altered significantly, receiving a number of new components made from mild steel. The turret lost its roof and the two machineguns were replaced with the cannon, covered by a massive mantlet. Otherwise, the tank was identical to the M2A1 Light Tank.
Trials began to evaluate the worth of the vehicle as a tank destroyer. Sadly, the results were unsatisfactory. The penetration of the 47 mm gun was barely above 30 mm at 728 meters. This was enough for now, but the military understood very well that it wouldn't be for long.
At the same time, work was underway on the 37 mm M3 gun, which surpassed the 47 mm gun in penetration. The idea of a tank support vehicle was abandoned, but the idea of using an open turret on a tank chassis was still attractive to the military.
The American military returned to the idea of a tank destroyer in late 1941. Work started in several directions, both on the chassis and on guns. The 37 mm M3 gun turned out sufficiently powerful and worked well against enemy tanks. It was decided to mount this gun on light wheeled and tracked tank destroyers.
Additionally, the US had many 75 mm M1897 field guns, a version of the French Canon de 75 mle 1897. Even though this gun was old, it was capable of fighting tanks.
Finally, the M3 76 mm AA gun was explored as an anti-tank method. The M5 AT gun was built based on this gun's excellent ballistics. It eventually migrated to tanks and to light and medium tank destroyers, but none of this happened immediately.
Initially, the 7 ton Cletrac tractor, indexed M2 High Speed Tractor in February of 1941, was selected as the base for the 76 mm tank destroyer. Work on the tank destroyer began in December of 1940, and in November of 1941, the T1 Gun Motor Carriage was ready for trials. The result was a very agile vehicle that could reach a speed of 60 kph.
The military was satisfied, and it was standardized as the M5 Gun Motor Carriage in January of 1942. Mass production was planned, but never happened for several reasons. The process of improving the M5 GMC to match requirements dragged on. While this happened, the T35 GMC entered trials, created on the chassis of the M4 medium tank. The M5 GMC had only one advantage over this vehicle: speed. At the same time, the M5's armour was only a gun shield, the horizontal traverse was very limited, and the ammunition capacity low. As a result, work on the M5 GMC was not continued past a prototype.
Even before the T1 GMC made it to trials, work began on a 76 mm tank destroyer on the Light Tank M3 chassis. Initiated in September of 1941, the M20 Gun Motor Carriage project envisioned a light tank destroyer with a low silhouette. The use of an already established light tank as a base meant shortening the road to production.
As the Light Tank M3 was equipped with a large radial Continental W-670 engine, it was replaced with a car engine. A pair of V8 Cadillac 42-series engines was used. Due to this solution, the M20 was lower than its donor chassis. The project as short lived, however, as discussion at a conference dedicated to self propelled artillery in October of 1941 decided that the M3 chassis was ill suited for the 76 mm gun. The project was closed, but the M3 chassis would soon be in demand again.
In December of 1941, the Bureau of Ordnance proposed a sketch of a converted M4 turret on the M3 light tank chassis. According to the results of the conference, the 75 mm M1897 gun was the maximum that the M3 chassis could handle. The project, indexed T29 Gun Motor Carriage, was closed in April 1942 due to low interest, but already in May, work on a new tank destroyer with a 76 mm gun on the M3 chassis began.
The first variant, indexed T50 Motor Gun Carriage, was an evolution of the T29 GMC. A turret very similar to that of the M4 was installed on a modified M3 chassis. Aside from thinner armour, the new turret had no roof or rear. Theoretically, after installation of the 76 mm gun, there would be enough room for a gunner/commander and loader. The second variant, dated late June 1942, used the same gun, but with traverse limited to 15 degrees in each direction horizontally. The result was very similar to the T1 GMC. Even though this design lacked a gun shield, it would certainly have been installed if such a vehicle was built.
These projects were rejected. Both T50 GMCs had a very high center of gravity. The military also remarked that the 76 mm gun is too big and powerful for the small chassis. The second variant also offered poor mobility of fire, which was already one of the reasons for closing the T1/M5 GCM project.
In July of 1942, Buick produced a prototype of the 57 mm T49 GMC tank destroyer. This vehicle had nothing in common with the M3 Light Tank, and further development led to the creation of the T70 GMC, better known as the M18 GMC, or "Hellcat". Compared to the T49, which the military liked very much, the T50 seemed miserly and without a future.
Third Time's the Charm
Bad luck with the T29 and T50 didn't spell the end for attempts to make a tank destroyer out of the M3. Work continued, but the chassis changed. In September of 1942, the American Car & Foundry Co. began building a new tank indexed Light Tank M3A3. The chassis remained the same, but the turret and hull were altered. The M3A3 was the basis of the next design. The Bureau of Ordnance first started talking about this new design on August 13th, 1942, when a draft of a 76 mm tank destroyer with a rear mounted gun was sent in. This layout was used based on results of T1 GMC trials. The proposal was approved, and, on September 16th, 1942, the Bureau of Ordnance began a project of a new tank destroyer. On the third try, attempts to make a tank destroyer on the M3 light tank platform finally reached the practical stage.
Two parallel projects were launched to test two different power plant method. The T56 and T57 GMC only differed in engines: the T56 received a 288 hp Continental W-670 12-series, a turbocharged version of the stock M3A3 engine, and the T57 received a 400 hp Continental R-975-C1 engine, used on the M4 medium tank. Due to the more powerful engine, air filters were moved outside of the hull.
The vehicles designed by the American Car & Foundry Co. were significantly different from most American SPGs on a tank chassis. Only the M12 was similar, and that was designed for indirect fire, whereas the target for T56/T57 was enemy tanks.
The chassis and front of the M3A3 were unchanged. The engine compartment was moved from the rear of the hull to the middle, forming space in the rear for the gun mount, ammunition, and crew. At first, no gun shield was installed. A special ramp was deployed behind the vehicle for the crew to work on. In other words, it was less a highly mobile tank destroyer and more of a motorized anti-tank gun.
The T56 was ready first, and shipped to the Aberdeen proving grounds in November of 1942. The trials began with some confusion. The problem was that the 40 round ammunition rack completely covered access to the engine from the rear. Access from the top was no easier, as the whole gun had to be removed. Service of the vehicle using the crew's own resources was problematic.
The placement of the engine compartment in the center of the vehicle made the driver's compartment very crowded. The driver and his assistant had to sit in a very cramped space, and they still had it better than the rest of the crew, as the T56 and T57 had no seats for them at all.
Mobility trials over 190 km showed a maximum speed of 62.5 kph. On the other hand, the vehicle was overloaded in the rear. Firing trials showed that the Bureau of Ordnance was right to insist that the M3 was too small for such a gun. Every shot threw the T56 back 35 cm.
A gun shield was installed on the T56 during trials, as well as optics, since the T56 came from the factory with no sights installed. Due to the design of the gun, it had to be aimed by two crewmen.
The T57 GMC arrived at Aberdeen on December 8th, 1942. The gun mount was altered according to experience from T56 trials. The T57 GMC drove 1419 km between December 17th, 1942 and April 13th, 1943. Thanks to a more powerful engine, it achieved the speed of 80 kph on a highway and 48 kph off-road. That was the only advantage of this tank destroyer, as most problems with the T56 didn't go anywhere.
On January 7th, 1943, the Bureau of Ordnance gave an order for 6 prototypes of the 76 mm T70 GCM, an evolution of the T49 design. The first T70 was finished right as the T57's trials concluded, and there wasn't much to show off against this new competitor. With the same top speed and armament, the T70 had a proper turret. It is not surprising that the program to develop a tank destroyer on the M3 chassis was closed.
Original article by Yuri Pasholok.