Initially specified before the outbreak of the Second World War the (General Staff designation) A20 was to be the replacement for the Matilda II and Valentine infantry tanks. In accordance with British infantry tank doctrine and based on the expected needs of World War I-style trench warfare, the tank was required to be capable of navigating shell-cratered ground, demolishing infantry obstacles such as barbed wire, and attacking fixed enemy defences; for these purposes, great speed and heavy armament were not required.
The vehicle was specified initially to be armed with two QF 2 pounder guns each located in a side sponson, with a coaxial BESA machine gun. A third BESA and a smoke projector would be fitted in the front hull. The specification was revised to prefer a turret with 60 mm of armour to protect against ordinary shells from the German 37 mm gun. Outline drawings were produced based on using the A12 Matilda turret and the engine of the Covenanter tank. Detail design and construction of the A20 was given to the Belfast shipbuilders Harland and Wolff who completed four prototypes by June 1940. During the construction period the armament was reconsidered which including fitting either a 6 pounder or a French 75 mm gun in the forward hull. In the end a 3-inch howitzer was chosen. The A20 designs were short-lived however, as at roughly the same time the emergency evacuation of the British Expeditionary Force from Dunkirk occurred.
At 43 tons, with a 300 hp flat-12 Meadows engine, the A20 had limited power compared to the 18-ton Covenanter. This was a less serious limitation than it might appear, owing to the British distinction between the high-speed cruiser tanks and the slow-speed infantry tanks. Vauxhall were approached to see if they could build the A20 and one example was sent to Vauxhall at Luton to see if they could provide an alternative engine. To this end they developed a flat-12 petrol engine. For speed of production, this engine was based on a Bedford six-cylinder lorry engine, giving rise to its name of "Twin-Six". Although still a sidevalve engine, the engine was developed with high squish pistons, dual ignition and sodium-cooled exhaust valves in Stellite seats to give 350 bhp.
With France lost, the scenario of trench warfare in Northern Europe was no longer applicable and the design was revised by Henry Merritt, Director of Tank Design at Woolwich Arsenal, based on the combat witnessed in Poland and France. These new specifications, for the A22 or Infantry Tank Mark IV, were given to Vauxhall in June 1940.
With German invasion of Britain looking imminent, and the loss of a substantial amount of military vehicles in the evacuation from France, the War Office specified that the A22 had to enter production within a year. By July 1940 the design was complete and by December of that year the first prototypes were completed; in June 1941, almost exactly a year as specified, the first Churchill tanks began rolling off the production line.
A leaflet from the manufacturer was added to the User Handbook saying:
Fighting vehicles are urgently required, and instructions have been received to proceed with the vehicle as it is rather than hold up production.
All those things which we know are not as they should be will be put right.
The document then described known faults, with work-rounds and what was being done to correct the problem.
Because of its hasty development, there had been little testing, and the Churchill was plagued with mechanical faults. Most apparent was that the Churchill's engine was underpowered, unreliable, and difficult to access for servicing. Another serious shortcoming was the tank's weak armament, the 2 pounder (40 mm) gun, which was improved by the addition of a 3-inch howitzer in the hull to deliver an HE shell, albeit not on a howitzer's usual high trajectory.
Production of a turret to carry the QF 6 pounder gun began in 1941, but problems with the plate used in an all-welded design led to an alternative cast turret also being produced. These formed the distinction between Mark III and Mark IV.
The poor performance of the Churchill nearly caused production to be ceased in favour of the forthcoming Cromwell tank; it was saved by the successful use of the Mk III at the Second Battle of El Alamein in October 1942.
The second major improved Churchill, the Mk VII, was first used in the Battle of Normandy in 1944. The Mk VII improved on the already heavy armour of the Churchill with a wider chassis and the British 75 mm gun which had been introduced on the Mk VI. It was primarily this variant, the A22F, which served through the remainder of war. It was re-designated A42 in 1945.
The Churchill was notable for its versatility and was utilized in numerous specialist roles.