M7 Priest

Battle reports in the early stages of WWII indicated the need for highly mobile artillery to accompany fast moving armored columns. It was realized this requirement could be fulfilled by a self-propelled mount on a fully-tracked chassis. In October 1941, two howitzer motor carriages based on the M3 medium chassis were manufactured. These pilots were designated as the T32 HMC and mounted the 105 mm M2A1 howitzer. The gun could traverse 15 degrees to the left and 30 degrees to the right, with an elevation range of +35 to -5.

With a few improvements, including lowing the silhouette, raising the frontal armor to 3 inches, and decreasing the side and rear armor to 1/2 inches, the T32 was standardized as the M7 HMC in April 1942. The M7 would gain the "Priest" nickname because of the pulpit appearance of the .50 caliber antiaircraft mount.

By the end of 1943, further improvements were made to the M7. As the M3 medium chassis grew out of favor, further M7s would be produced on the M4A3 design. These vehicles were designated as M7B1. These vehicles were similar to late model M7s. The M7 would go on to see over 2800 produced, rapidly replacing other 105 mm howitzer mounts and see further use in the Korean War.

M7B1

In June 1942, a project was started to adapt the British 25-pounder Mk. II gun. The second T32 pilot was reworked to mount the British weapon and the vehicle was redesginated as the T51 GMC. However, the successful development of the Canadian Sexton killed further interest in this project.

What I've done for this tank is modify its weapon and engine choices for more historical options. 

Guns: 105 mm Howitzer M2A1; 25-pdr Mk. II

Engines: Continental R-975-C1 (350 hp); Ford GAA (500 hp)

Suspensions/Alternate Hulls, Total Weight (metric tons): M7 (22.95); M7B1 (??)

Hull Armor (mm): 76/13/13 (M7); 38/13/13 (M7B1)

Top Speed: 34 km/h (M7); 42 km/h (M7B1)

Crew: 6 (Driver; Gunner; Loader; Loader; Commander; Radio)

Parent: T1 HMC
Children: M37

Sources:

Hunnicutt, R. P. Sherman. 2015.