A replacement was needed for the M12 GMC (Gun Motor Carriage) as the supply of M1918 guns was exhausted and it's chassis was unsuitable for the more powerful M1 and M2 155 mm guns.
Used M4A3 chassis that was widened and used the high volute suspension springing (HVSS). The engine was moved to the front and a spade was placed in the rear to help absorb some of the recoil.
Front Crew Compartment
The M40 had a compartment in the front that the driver sat in. Beside him was the co-driver. The M40 had an escape hatch located just behind the co-driver.
The driving compartment had 2 hatches in the roof for access.
Behind the crew compartment was the engine.
The M40's main armament was located in the rear of the vehicle.
Gun had range of 25,175 yards with a 95 lb round.
Rear Fighting Compartment
The gun crew would stand on a hinged platform when firing the gun.
Intended to be used well behind the front lines so no protection for the fighting compartment was provided.