The A20 was designed to meet the expected needs of First World War style trench warfare, where the main emphasis of the tank was to navigate shell cratered ground and demolish infantry obstacles such as barbed wire and was influenced that for the French Char B tank. Between then the Woolwich Arsenal and Harland & Wolff developed the A20 from a specification in 1939 to working prototype form by June 1940. Once had been complete Vauxhall (the car manufacturers) took over the project, designing and building a prototype, which was designed the A22 by November 1940. The later version of tank had evolved based on the battles in Poland and France. However it still kept many of the same features that would put it a disadvantage confronted with the rapid nature of blitzkrieg tactics but were also prove to be some of its strengths. The first production units were available by the middle 1941.

The hull was made up of simple flat plates initially bolted but later welded together. The suspension was fitted under the two large "panniers" either side of the hull - the track running over the top. There were 11 bogies either side, each carrying two 10-inch wheels. Only 9 of the bogies were taking the vehicle weight normally, with the front ones only coming into play when the vehicle nosed into the ground or against an obstacle. The rear ones acted in part as track tensioners. The twin engines were connected through a common crankshaft feeding a regenerative transmission steered by a tiller bar rather than levers or steering wheel. The interior was comparatively roomy and the large hatches in the sides made escape easy. They were also to be one of the reasons for its later conversion into the AVRE. The first turrets were cast with a rounded shape - sufficient for the relatively small 2-pdr gun, but when a larger gun (6-pdr) was required the turrets became larger with welded construction. To fulfil its role as an infantry support vehicle the first models were equipped with a 3 inch howitzer in the hull and although this could deliver a useful HE round complementing the weakness of the 2-pdr in that area it was limited by a poor fire arc, due to the way the tracks extended in front of the hull.

The hurried production in light of a possible invasion and lack of field tests, meant that the tank entered service while still suffering from mechanical problems and defects. This meant that it performed poorly in its first combat outing, which was the disastrous raid on Dieppe in 1942. After numerous modifications, the tank did begin to see better performance in the North African Campaign though, where its exceptionally heavy armour, low silhouette and good climbing abilities gave it a reasonable degree of success. However, main complaints against it were that is was very low speed and had poor armament. These two weaknesses that would haunt it throughout its entire career, but after some modifications the Churchill tank served later in both the Italian Campaign and the Western Front.