The Avro Type H, Type 501, and Type 503 were a family of early British military seaplanes. They were a development of the Avro 500 design and were originally conceived of as amphibious; the prototype being fitted with a single large main float (equipped with wheels) under the fuselage, and two outrigger floats under the wings. Tests were conducted on Lake Windermere in January 1913. It was later converted to twin-float configuration and bought by the British Admiralty. It now, however, proved too heavy and was converted again - this time to a landplane.
An improved version, designated the 503 was demonstrated for the Inspector of Naval Aircraft, who placed an order for three machines. The prototype itself was demonstrated for the German Navy in its seaplane trials in June 1913 and was purchased by the government of Imperial Germany for evaluation purposes. This machine subsequently became the first aircraft to fly across the North Sea, from Wilhelmshaven to Heligoland, in September 1913. Gotha purchased a licence from Avro and produced the type as the WD.1 (Wasser Doppeldecker - Water Biplane). Unlicenced copies were also built by Albatros, AGO, Friedrichshafen. Some WD.1s were provided to the Ottoman Empire following their withdrawal from German Navy service.