Prior to World War 2, the German Navy owned about 60 "U-Boat" submarines that posed a great threat to Britain, who mainly relied on seaborne supplies of goods. At the outset of the war, several destroyers were in service for the Royal Navy; such as the "A" Class (which were built during the 1920's) to the K Class completed in 1939, and the larger, Tribal Class destroyers, buth they were far from being sufficient in number. There became a pressing need to construct new destroyers and escort transport vessels in a very short period of time. Based upon the First Emergency Flotilla program announced in 1939, the Onslow, the leader of the "O" class destroyers, was laid down in 1940. She entered in service in mid-1941, and seven more ships of her class were completed by the end of 1942.
