British Heavy Assault Tank - A39 Tortoise. In the early part of 1943 a new class of vehicles designed for British army, in the shape of Assault tanks, was developed to resist German fighting from heavily fortified positions. In response to the Joint Memorandum issued by the Secretary of State for War and the Minister of Supply, the Nuffield Organization came out with 18 separate designs (AT1 through AT18) drafted between May 1943 and February 1944, each design being larger and heavier than the