Design work began at John I. Thornycroft Ltd. in May 1938 with trials completing in February 1940. Although early LCM(1)s were powered by two Thornycroft 60 bhp petrol engines, the majority were powered by Chrysler, in-line, 6-cylinder Crown petrol engines. Constructed of steel and selectively clad with armour plate, this shallow-draft, barge-like boat with a crew of 6, could ferry a tank of 16 long tons to shore at 7 knots (13 km/h). Depending on the weight of the tank to be transported the craft might be lowered into the water by its davits already loaded or could have the tank placed in it after being lowered into the water.