[Would you like to see this in-game?]
TL;DR: An experimental version of the T3 HMC with a 37mm cannon mounted on top of the 75mm howitzer.
Note: This is the only picture I could find of this tank. If you have any other significant information, please let me know.
The need to motorize artillery was felt by the US military as early as WWI. Many projects attempted to satisfy this need, however, most ended in rejection. The T3 HMC is one such rejection, which was based on the lessons learned from the T1 HMC, a HMC based on the T1 Cunningham. At the time of the T3 HMC’s development, the US had finally begun mass tank production. One of the tanks produced en masse was the M1 Combat Car, a lightweight and incredibly quick tracked vehicle. These characteristics made the vehicle very appealing to engineers as a possible platform for a mobile artillery piece. In 1939, construction of a pilot vehicle was approved and the T3 HMC was born. This vehicle featured a the M1A1 75mm pack howitzer as its main weapon, mounted on the right side of the fighting compartment, while a turret equipped with a .30cal MG occupied the left. Only two men could fit in the small SPG. This resulted in very odd crew roles. The driver was responsible for aiming and firing the weapon and the commander was responsible for loading, while also being in control of the MG turret. These roles made firing on the move incredibly difficult. The vehicle went through trials at Aberdeen before being shipped over to Fort Bragg where it underwent more testing. One of these tests involved the the mounting of a M1916 37mm cannon to the top of the howitzer just to see if the driver could aim it. Of course, these tests didn’t yield good results and the 37mm was dismounted. The T3 HMC project was later abandoned due to being unsuitable for any role.
Place In War Thunder:
Quick derp vehicles have always had their fair share of fans. Multi-gun vehicles have always had their fair share of fans. What happens when you combine the two vehicle types? You get the T3 HMC (75/37), a vehicle equipped with two very different cannons, both of which are very limited in use. You need to be at close range to effectively use the 75 and you might as well be shooting peas with the 37. That being said, usefulness isn’t the only thing that matters. Vehicles that are simply just dumb fun are also necessary. Playstyle would be rather unique due to its unique armament. Your speed would easily allow you to get close enough to use your howitzer and your 37 could act as a support weapon, quickly pelting whatever is in front of you. Due to its uniqueness, I think that the best implementation for this vehicle is as a low tier GE premium or event vehicle. It was an incredibly experimental variation of an already experimental vehicle. It has no place in the tech tree.
Armament: M1A1 75mm howitzer, M1916 37mm cannon, and 1x .30 cal MG
Dimensions: 4.14m, 2.40m, 2.26m (L,W,H)
Armor: 16mm max
Ammunition: 75mm: HEAT and HE 37mm: HE and LE
Tank Archives: T18 HMC: Quick Howitzer
Off The Mark: US Revamp: T3 HMC
M1 Combat Car - Wikipedia
M116 howitzer - Wikipedia
Stuart: A History of the American Light Tank Vol. 1 by R.P. Hunnicutt