The Canadian Sexton Self Propelled Gun (SPG) was constructed to utilize a 25-pound round, the same ammunition being used as a standard caliber in the British Army. During World War 2 the British "Tommy's" were using the American M7 "Priest" 105mm self-propelled howitzer across Africa in actions against the Germans. This particular use of American weaponry required the British Army to be specifically supplied with the special 105mm shells instead of operating from the standard supply lines marked between the forward positions and British supply depots. Additionally, time was also needed to move the ammunition for the M7 from American supply depots themselves, before even arriving to British units. In 1942, the British War Department requested the US Army to fit the 25 pound gun to the M7 chassis - this was tried but failed during test-firing of the 25 pound gun.
At the same time, the US Army and the Canadian Government's Department of Munitions and Supply were designing an SPG version using the 25-Pounder gun on an American M3 Lee medium tank body. The prototype was completed and shipped to Britain for trials. The 25-pounder underwent firing trials and was found to be suitable to the British needs, who in turn ordered 300 units in May of 1943 and designated it as the "Sexton". The first order of 300 would be built on the "Grizzly" medium tank body, essentially a Canadian version of the American M4A1 Sherman medium tank, designated as the "Sexton Mark II" in the Canadian Army. The remainder would be built "Ram" medium tank hulls and would be identified as the "Sexton Mark I".
All the Sextons were built in Canada at the Montreal Locomotive Works, this facility producing 2,150 SPG's between 1943 and 1945 for both Canadian and British units. The British 8th Army received their first Sexton's in 1943 just in time for the battle of Italy. During the D-day landings, a number of Sextons fired at German shore positions from their landing craft as they approached the beaches, those these had little effect for they were not very accurate from the water. Later, during the Battle of Normandy and the campaign in North Western Europe, the Sextons were assigned to tank battalions to provide a limited anti-aircraft role as well as support in direct fire missions against enemy infantry.
At times, the Sexton would be used as a howitzer for "plunging fire" (arc firing on the enemy) and, other times, as a gun platform for direct fire missions. While working with British artillery units in France, the Sexton was used to attack company-sized units over a series of hills between friendly forces. For this mission, a battery of Sextons had their 25-pounders elevated to the maximum firing position and, using minimum charges, were able to lob the shells as plunging fire over the hills and onto enemy infantry forces. The Sexton was also used for hit-and-run missions as in Holland when the Sextons moved up under the cover of darkness and shelled German positions, only to then retreat back to British lines before German artillery could return fire. This tactic was used again and again with success during indirect fire support missions.
The Sexton was a successful design through and through. She was fitted with the Continental R-975 air-cooled, 9-cylinder radial engine of 400 bhp. She could achieve 25mph on paved roads though less than that on off road terrain. The system carried a crew of six personnel and was primarily armed with her single 25-pounder field gun and normally carried 2 x Bren .303 caliber light machine guns to provide some close-in anti-aircraft defense as well as fire support against advancing enemy troops. Stowage lockers were provided on board for ammunition and personal gear. A canvas cover was afforded the crew in case of bad weather (the Sexton maintained an open turret). Camouflage netting was also provided to help conceal the Sexton from searching enemy aircraft.
The Montreal Locomotive Works built 2,510 Sextons of all types and a number remained in British service up until 1956.