The Tortoise heavy assault tank (A39) was a British heavy assault gun design developed during the Second World War, but never put into mass production. 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. Although heavy, at 78 tons, and not readily transported, it was considered reliable and a good gun platform. Only a few prototypes of the Tortoise had been produced by the end of the war. After testing was complete, one was retained for preservation and the others disposed of. Development 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. Initially, work was concentrated on the Excelsior tank (A33), based on the Cromwell tank. There was also a programme to upgrade the armour of the Churchill tank. For similar work in the Far East, the Valiant tank (A38), based on the Valentine tank was considered, although weight was specified to be as low as possible. 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. The Nuffield Organization responded with 18 separate designs (AT1 through AT18) drafted between May 1943 and February 1944, each design larger and heavier than the last. 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. An order for 25 was placed by the War Office and work was begun. Following the end of the war the order was reduced and only six vehicles were built. One example was sent to Germany for trials, where it was found to be mechanically reliable and a powerful and accurate gun platform. However, at a weight of 80 tons and a height of it was extremely slow