HMS Ark Royal (pennant number 91) was an aircraft carrier of the Royal Navy that served during the Second World War. Designed in 1934 to fit the restrictions of the Washington Naval Treaty, Ark Royal was built by Cammell Laird and Company, Ltd. at Birkenhead, England, and completed in November 1938. Her design differed from previous aircraft carriers. Ark Royal was the first ship on which the hangars and flight deck were an integral part of the hull, instead of an add-on or part of the superstructure. Designed to carry a large number of aircraft, she had two hangar deck levels. She served during a period that first saw the extensive use of naval air power; a number of carrier tactics were developed and refined aboard Ark Royal.