[1][page needed] All three were originally American type C3 merchant ships in the process of being built at the Sun Shipbuilding and Drydock Company Chester, Pennsylvania.

The ships laid down in 1939 and 1940 were launched and delivered to the Royal Navy by 1942 under the Lend-Lease agreement.

Propulsion was provided by four diesel engines connected to one shaft giving 8,500 brake horsepower (6,300 kW), which could propel the ships at 16.5 knots (30.6 km/h; 19.0 mph).

Aircraft could be housed in the 190 by 47 feet (58 m × 14 m) half hangar below the flight deck.

A fourth ship, USS Charger was built at the same time to the same design but was commissioned in the U.S.