The self-propelled artillery piece (SP), marrying the fire-power of the field gun to the mobility of the tank, first appeared during the Great War and was developed further during the inter-war years. The Germans used their Sturmgeschutz ("assault gun") for direct infantry support early in the Second World War, but then realised the value of the mobile gun in an anti-tank role and discontinued production of the Sturmgeschutz in favour of various models of Panzerjäger ("tank destroyer"). One might well ask why more tanks were not simply produced, considering that the German tanks were able to mount guns, in the Panthers and Tigers at least, which were more than capable of destroying the best tanks the Allies had by the time of the Normandy campaign. A key factor was the relative expense of the tank as compared to the SP, which, lacking a fully-traversing turret, could be manufactured more cheaply and in larger numbers.

As for the British Army, the SP remained first and foremost a field artillery piece. Royal Artillery doctrine since the First World War was governed by adherence to the principle of concentration of fire-power. Any suggestion of decentralization of control of the guns into 'penny packets' was stubbornly attacked. To use SPs in the manner of the Sturmgeschutz would have meant apportioning small numbers of guns to forward units, and depriving the Royal Artillery of its devastating ability to bring large numbers of guns to bear on a single target in a short time.

The British SP field gun was not designed for firing on the move and was not armed with machine-guns, so it could not defend itself against enemy infantry. It was thought that a suitable role would be to engage tanks that stood off and neutralized forward localities. SPs like the Sexton 25-pounder were therefore never expected to provide direct fire-support to tanks on the leading edge of an assault. They were used in the Canadian and British armies as conventional towed pieces, deployed in indirect fire positions well behind the tanks. Gunners providing indirect support could not see the target, but had their fire corrected by a Forward Observation Officer (FOO).

Sexton SP guns of the 8th Field Regiment, R.C.A., firing 21 gun salute to celebrate VE Day, Groningen, Netherlands, 8 May 1945. Photo by Jack H. Smith. Department of National Defence / National Archives of Canada, PA-114372.

Sexton 25-pounder self-propelled gun

Entering service in September 1943, the Sexton SP mounted the 25-pounder gun on the Canadian Ram chassis and was built at the Montreal Locomotive Works. The Sexton was driven from the right-hand position, as were all vehicles produced for the British and Canadian armies. A number of gun modifications had to be made to fit the 25-pdr to the Sexton, such as limiting of the recoil system so that the gun could be properly elevated. Sextons were widely used within 21st Army Group during the campaign in Northwest Europe after July 1944 when the 25-pdr replaced the American 105 mm SP, called the "Priest". The conversion to Sextons rendered the 2nd Canadian Corps' Priests superfluous, and the latter were modified to be used as Kangaroos during Operation Totalize, 7-10 August 1944, in what was the first practical use of the armoured personnel carrier in combat.