It was developed for the task of clearing heavily fortified areas such as the Siegfried Line and as a result favoured armour protection over mobility.

In the early part of 1943, the Allied forces anticipated considerable resistance in the projected future invasion of Europe, with the enemy fighting from heavily fortified positions such as the Siegfried Line.

As a result, a new class of vehicles emerged, in the shape of assault tanks, which placed maximum armour protection at a higher priority than mobility.

The Secretary of State for War (James Grigg) and the Minister of Supply (Andrew Duncan) issued a Joint Memorandum in April 1943 that gave a vague specification for an assault tank, classing it as a special purpose vehicle to operate in heavily defended areas as part of the specialist 79th Armoured Division.

By February 1944, design AT16 was complete and was approved by the Tank Board, who proposed that month that 25 should be produced directly from the mockup stage without bothering with a prototype, to be available for operational service in September 1945.

This led to Tortoise having very thick armour, arranged as a one piece casemate to avoid the weak spots of a turret design.