Reflecting the design concept of British flying-boats that had originated in the 1920s, the Sarņ London twinengine biplane was an all-metal aircraft with fabric-covered wings and tail, and a metal-skinned hull. The type served with RAF Coastal Command during the first two years of World War II. Designed to Air Ministry Specification R. 24/31, the prototype first flew in 1934 with two 559kW Bristol Pegasus III radiais, the engines being mounted on the top wing to be well clear of spray while taking off and landing; the prototype went on to serve for periods between 1934 and 1936 with Nos 209 and 210 Squadrons at Felixstowe and Gibraltar. Production deliveries started in March 1936 with Pegasus III engines, but from the eleventh aircraft the Pegasus X was fitted and the aircraft's designation changed to London Mk II, this variant equipping Nos 201 and 204 Squadrons in 1936 at Calshot and Mount Batten repectively. In 1937 they joined No. 202 Squadron at Kalafrana (Malta) and No. 228 Squadron at Pembroke Dock respectively. By the outbreak of war in September 1939 Londons still equipped No. 201 Squadron, then at Sullom Voe in the Shetland Islands, and No. 202 Squadron still at Gibraltar, while No. 240 Squadron had re-equipped with Londons in July 1939 and was stationed at Invergordon, These flyingboats carried out sea patrols over the North Sea and the Mediterranean, some aircraft being fitted with a large dorsal fuel tank to increase their range. Bombs, depth charges and (occasionally) mines up to a total weight of 907kg could be carried under the lower wing roots. Indeed, the old biplanes undertook a considerable share of the patrol work over the North Sea, keeping watch for the likely breakout into the Atlantic by German surface and submarine raiders as well as the return to German port by blockade runners. Gradually maritime reconnaissance aircraft such as the Lockheed Hudson came to assume these responsibilities while Short Sunderland flying-boats equipped the squadrons flying over the Atlantic and Mediterranean. The Londons were replaced on No. 201 Squadron in April 1940, followed two months later by those on No. 240 Squadron. Only No. 202 Squadron continued to fly Londons at Gibraltar until June 1941.