The M40 155mm Gun Motor Carriage was a self propelled gun based on a modified M4A3 Sherman chassis. It arrived too late to see significant service during the Second World War, but as used in large numbers during the Korean War.
The M12 155m Gun Motor Carriage had been designed in 1941 and tested in 1942. After some debate within the US Army a total of 100 were ordered in two batches, with the final deliveries in March 1943. Army Ground Forces, the section of the US Army responsible for the home-based development of the army, was never keen on the M12, and after being built most of them went to training units or into storage.
Late in 1943 the Artillery decided that it wanted the M12 for the upcoming invasion of Europe and in December work began on modernizing the M12. The Artillery also requested additional M12s, but supplies of the French M1917 and M1918 155mm guns used in the M12 had now run out. The American M1 155mm gun was heavier and more powerful, and so on January 1944 work began on a stronger vehicle that could cope with the newer gun.
The new vehicle, with the designation T83, was largely based on the M4A3 Sherman medium tank. It used the horizontal volute spring suspension introduced on later Shermans, and had a wider hull than the gun tank. The T83 used 23in wide tracks. As on the M12 the engine was moved forward to make space for the gun compartment. The drivers stayed in their normal position in the front of the vehicle. A new open fighting compartment was designed, with a folding rear wall that could be lowered to serve as a fighting platform.
Production of five pilot T83s was authorized on 18 March 1944. The first pilot was completed on 28 July 1944 and went to the Aberdeen Proving Grounds for tests. The tests with the 155mm gun were successful. The 155mm gun was then replaced with an 8in howitzer, which also performed well. The fourth and fifth pilots were completed as the T89 8in Howitzer Motor Carriage, and became the prototypes for the M43 8in Howitzer Motor Carriage.
The second and third T83 were completed in October and were used for tests at the Field Artillery Board, Fort Bragg. A number of changes were made after these trials, and production began in February 1945. A total of 418 T83s were delivered during 1945, and in May it was standardized as the M40 155mm Gun Motor Carriage. Twenty four were later converted into M43s.
The M12 had been designed to be used alongside the M30 Cargo Carrier, which was an M12 with the gun removed. The M40 was also produced as a cargo carrier, the T30, also produced by removing the gun. Very few T30s were built.
The M40 made its combat debut early in 1945. One of the T83 pilots was taken to France as part of the Zebra mission, along with one T89 and a number of Pershings. The T83 and T89 went to the 991st Field artillery Battalion, which promptly replaced the 8in howitzer with a 155mm gun. The two 155m armed vehicles then took part in the battle for Cologne. The T89 later had its howitzer restored and was tested in that configuration.
One lesson of this first test was that the open fighting compartment left the gun crews very exposed to enemy fire. General Barnes, commander of the Zebra mission, suggested that some effort should be put into produced some sort of armour to protect the crews, and also that secondary machine gun armament be installed. An armoured cab was designed, but it wasn’t considered suitable.
A handful of M40s reached the front where they replaced worn-out M12s, but their main combat use came during the Korean War, where both the M40 and M43 proved to be invaluable during the long stalemate on the 38th Parallel. After the Korean War the M40 was withdrawn from front line service in the US Army. A handful of M40s were used by the British after the Second World War.
Hull Length: 20ft 7in (29ft 9in including gun)
Hull Width: 10ft 4in
Height: 8ft 9 1/2in
Crew: 8 (commander, driver, six gun crew)
Engine: Continental R-975 radial engine
Max Speed: 25mph road, 20mph cross-country
Max Range: 107 miles road radius
Armament: One 155mm M2 gun