The Shelled Area Infantry Tank A20 was a heavy tank designed to operate in the muddy no-mans-land that might have developed between the Maginot and Siegfried Lines, but that was cancelled due to a combination of its own poor performance and the German blitzkrieg of 1940.
As war began to loom in 1939 the British General Staff began to worry about the sort of battles the army might have to fight. Existing tanks were designed for mobile warfare, but the French had their Maginot Line and the Germans the Siegfried Line and it was possible that the war might instead turn into another muddy stalemate, similar to the trench warfare of 1914-18.
In September 1939 the first specifications for a tank capable of operating in areas with soft muddy ground and under enemy artillery fire were developed. The new tank would have to be able to withstand the current 37mm German anti-tank gun, operate on soft ground, have a speed of 10mph and be transportable by rail. It would be used to carry out direct assaults on German fortifications. The first specification had no turret, sponsons on each side each armed with a 2-pounder gun and a coaxial Besa machine gun, and used supplementary tracks under the hull.
This design was examined by Vickers and the Mechanisation Board. The secondary tracks were quickly dropped. The required amount of armour wasn't compatible with the low ground pressure requirements, so the specification was changed to 60mm armour, which could resist 37mm artillery shells, but not 37mm anti-tank shells. The sponsons were abandoned in place of a turret.
The final specification called for a tank with 60mm armour, using the turret and armament from the A12 Matilda II, and the engine and transmission of the new Covenanter cruiser tank (not a good choice, as that tank proved to be stunningly unreliable). The outline drawings were ready by October 1939, and Harland and Wolff of Belfast were asked to build the new tank. The original plan was to skip the prototype stage and go straight into production, but in January 1940 this risky plan was abandoned. Instead Harland and Wolff were to built two pilots in soft steel, and production of the 100 tanks wasn't expected to be complete until 1941.
The project was plagued by a number of arguments. The original plan had been to use a new Harland and Wolff diesel engine, but this wouldn't be ready in time. The Meadows flat 12 of the Covenanter wasn't a popular choice, and so early in 1940 Vauxhall was asked to join the project. They produced a flat 12 petrol engine based on the Bedford truck engine. The engine would drive a Merritt-Brown combined gearbox and steering system. Suspension was probably the same as on the A7 medium tank, with fourteen pairs of small wheels on each side.
The main armament also caused a number of disputes. The Matilda II was armed with a 2-pounder gun, but newer weapons were already underway. The British 6-pounder and a French 75mm howitzer were both considered. Neither would fit into the turret, so the plan was for the main gun to be carried in the hull front. The 6-pounder was rejected because the barrel was too long and would be vulnerable to damage, the French 75mm gun because the hull front would need redesigning, a rather odd argument in an incomplete design. Eventually a 3in howitzer was chosen as the main gun.
The first pilot, the A20E1, was ready for trials in May 1940. At this point the design was considered to be acceptable, despite problems with the gearbox, and work continued. The Matilda turret was installed, and the weight of the vehicle rose to 40 tons.
The second pilot went to Vauxhall at Luton, but what happened to it there isn't clear.
The events of May 1940 showed that the fears of another stalemate on the western front were sadly untrue and the entire reason for producing the Shelled Area Infantry Tank disappeared. As a result the project was cancelled.
Visually the A20 rather resembled the later Churchill tank, which was also a heavily armoured tank, with similar looking suspension and a Bedford engine. The resemblance isn't surprising, as Vauxhall were made the lead firm on the Infantry Tank Mark IV Churchill (A22), and the specifications were somewhat similar.