High Quality Content by WIKIPEDIA articles! The Tank, Heavy Assault, Tortoise (A39) was a British heavy assault tank design developed in World War II but never put into production. It was developed for the task of clearing heavily fortified areas 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. Initially, work was concentrated on the Excelsior tank (A33), based on the Cromwell tank, and the Valiant tank (A38), based on the Valentine tank. There was also a program to upgrade the armour of the Churchill tank.