The development of this tank started in 1941 when a Tetrarch light tank had a Canvas screen fitted to the outside of the tank. This screen was made from waterproof Canvas and could be raised and lowers from inside the tank by the crew using a air pump and an inflatable rubber frame.The addition of the screen acting as a hull, increased the tanks freeboard, the distance between the water level and the top of the hull, allowing it to float. Around 460 Vickers Valentine were also converted to DD tanks before the decision was made to use the Sherman instead. This was partly due to the US armies unwillingness to use British due to training issues, and partly due to the Sherman being able to point its turret forward when the screen was up.This meant the tank was ready to fire as soon as it lande