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With France lost, the scenario of trench warfare in Northern Europe was no longer applicable and the design was revised by Henry Merritt, Director of Tank Design at Woolwich Arsenal, based on the combat witnessed in Poland and France.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.
Due to 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.
The hull was made up of simple flat plates which were initially bolted together and were welded in later models. The hull was split into four compartments: the driver's position at the front, then the fighting compartment including the turret, the engine compartment, and the gearbox compartment. The suspension was fitted under the two large "panniers" on either side of the hull, the track running over the top. There were eleven bogies either side, each carrying two 10-inch wheels. Only nine of the bogies were taking the vehicle weight normally, the front coming into play when the vehicle nosed into the ground or against an obstacle, the rear acting in part as a track tensioner. Due to the number of wheels, the tank could survive losing several without much in the way of adverse effects as well as traversing steeper terrain obstacles. As the tracks ran around the panniers, escape hatches in the side could be incorporated into the design. These were retained throughout the revisions of the Churchill and were of particular use when the Churchill was adopted as the AVRE.
The Bedford Vehicles engine was effectively two engines in horizontally opposed configuration ("flat twelve") on a common crankshaft. There were four Solex carburettors each on a separate manifold that fed three cylinders formed as a single cylinder head. The elements of the engine and ancillary components were laid out so they could be reached for maintenance through the engine deck covers. Air for the engine was drawn from the fighting compartment through air cleaners. Cooling air was drawn into the engine compartment through louvres on the sides, across the radiators and through the engine compartment by a fan driven by the clutch. This fan blew the air over the gearbox and out the rear of the hull. By opening a flap between the fighting compartment and the engine compartment this airflow could be used to remove fumes produced by firing the armament. The 1,296 cu in (21.238 L) capacity engine was rated at 350 bhp at 2,000 rpm delivering 960 lb·ft (1,300 N·m) over an engine speed range from 800 to 1,600 rpm.
The gearbox featured a regenerative steering system that was controlled by a tiller bar instead of the more commonplace brake levers, or as with the German Tiger I heavy tank, a steering wheel. The tiller was connected with servo assistance, hydraulically to the steering brakes. The Churchill was also the first tank to utilise the Merritt-Brown gearbox, which allowed the tank to be steered by changing the relative speeds of the two tracks; this effect became more pronounced with each lower gear, ultimately allowing the tank to perform a "neutral turn" when no gear was engaged, where it could fully pivot within its own length. There were final reduction gears, of the planetary type, in the driving wheels.
The first turrets were of cast construction and were rounded in shape, providing sufficient space to accommodate the relatively small 2 pounder gun. To fulfil its role as an infantry support vehicle the first models were equipped with a 3 inch howitzer in the hull in a layout very similar to the French Char B. This enabled the tank to deliver a useful high-explosive capability while retaining the antitank capabilities of the 2 pounder. However, like other multi-gun tanks, it was limited by a poor fire arc - the entire tank had to be turned to change the aim of the hull gun. The Mk II dispensed with the howitzer and replaced it with a bow machine gun and on the Mk III, the 2 pounder was replaced with the 6 pounder, significantly increasing the tank's anti-tank capabilities. The tank underwent field modification in North Africa with several Churchills being fitted with the 75 mm gun of destroyed M4 Shermans. These "NA75" variants were used in Italy. The use of the 75 mm, which was inferior as an anti-tank weapon to the 6 pounder but better as an all-around gun, was soon made standard on successive versions.
Churchills made use of the Vickers Tank Periscope MK.IV. In the Mark VII, the driver had two periscopes as well as a vision port in the hull front that could be opened. The hull gunner had a single periscope as well as the sighting telescope on the BESA mounting. In the turret the gunner and loader each had single periscope and the commander had two fitted in his hatch cupola.
The armour on the Churchill, often considered its most important feature, was originally specified to a minimum of 16 millimetres (0.63 in) and a maximum of 102 millimetres (4.0 in); this was increased with the Mk VII to a range from 25 millimetres (0.98 in) to 152 millimetres (6.0 in). Though this armour was considerably thicker than its rivals (including the German Tiger I tank, but not the Tiger II) it was not sloped, reducing its effectiveness. Earlier models were given extra armour by the expedient of welding extra plates on.
On the Mark VII, the hull front armour was made up of a lower angled piece of 5.5 in (140 mm), a nearly flat 2.25 in (57 mm) plate and a vertical 6 inch plate. The hull sides, were for the most part, 3.75 in (95 mm). The rear was 2 in (51 mm) and the hull top 0.525 in (13.3 mm). The turret of the Mark VII was 6 in (150 mm) to the front and 3.75 in (95 mm) for the other sides. The turret roof was 0.79 (20 mm) thick. Plate was specified as IT 80, the cast sections as IT 90.
The A22F, also known as "Heavy Churchill" was a major revision of the design. The most significant part was the use of welding instead of riveted construction. Welding had been considered earlier for the Churchill but until its future was assured this was no more than testing techniques and hulls at the firing ranges. What welding reduced in the overall weight (estimates were around 4%), the thicker armour of the A22F made up for. Welding also required fewer man-hours in construction. The hull doors changed from square to round which reduced stresses. A new turret went with the new hull. The sides, which included a flared base to protect the turret ring, were a single casting while the roof which did not need to be so thick was a plate fitted to the top.
Since the engines on the Churchill were never upgraded, the tank became increasingly slower as additional armour and armament was equipped and weight increased; while the Mk I weighed 39,120 kg (40 long tons) and the Mk III weighed 39,630 kg, the Mk VII weighed 40,640 kg. This caused a reduction in maximum speed of the tank from its original 26 km/h (16 mph) down to 20.5 km/h (12.7 mph). The engines also suffered from many mechanical problems.
Another problem was the tank's relatively small turret that prevented the use of powerful weapons; definitive versions of the tank were armed with either the QF 6 pounder or the derivative QF 75 mm gun. The 6-pdr was effective against armoured vehicles but less so against other targets, the 75mm a better all-rounder but lacking against armour. Although the Churchills with their 6 pounders could outgun many contemporary German medium tanks (like the Panzer IV with the short-barrel 75 mm gun and the Panzer III's 50 mm gun) and the thick armour of all Churchill models could usually withstand several hits from any German anti-tank gun, in the later years of the war the German Panther tank had a 75 mm high-velocity cannon as its main armament along with increased protection, against which the Churchills' own guns often lacked sufficient armour penetration to fight back effectively.
The Churchill had many variations, including many specialised modifications. The most significant change to the Churchill was that it was up-gunned from 2 pounder to 6 pounder and then 75 mm guns over the course of the war. By the war's end, the late model Churchill Mk VII had exceptional amounts of armour – considerably more than the German Tiger tank. However, the firepower weakness was never fully addressed. The Mark VII turret that was designed for the 75 mm gun was of composite construction – cast with top and bottom plates welded into position.
It is important to note that, despite its weaknesses, the Churchill had a significant advantage that was apparent throughout its career. Due to its multiple bogie suspension, it could cross terrain obstacles that most other tanks of its era could not. This feat served well, especially during the fighting in Normandy particularly the capture of Hill 309 between the 30 and 31 July 1944 in Operation Bluecoat conducted by VIII Corps.