The Terrapin 8 x 8 Amphibian was designed by John I. Thornycroft & Co. Ltd. to a specification from the War Office for an amphibious vehicle with a limited life that could be employed for a period of a single operation. The vehicle was intended to supplement the American 6 x 6 DUKW amphibian, which was only expected to be available to the British in limited numbers. As it was not possible for the British to divert any existing 6 x 6 lorry chassis production for conversion to the amphibious role, Thornycrofts had to display considerable ingenuity in order to evolve a concept that integrated off-the-shelf engines, gearboxes, and worm drives in a chassisless hull that carried a heavier payload than its American rival. The resulting Terrapin Mk.I offered advantages and disadvantages in comparison to the DUKW, and proved successful in its one major operation. The design was then evolved into the superior Terrapin Mk.II, which despite holding considerable promise, was shelved due to the increasing availability of the DUKW and the encroaching end of hostilities.