The original Avro Trainer of 1929 was designed as a replacement for the Avro 504N and featured a welded steel-tube structure and a 115kW Armstrong Siddeley Mongoose IIIA radial engine. Twenty-two were built for the RAF and three for the Irish Army Air Corps. Following evaluation Avro began mass production of the 621 for the RAF under the new name of Tutor. It eventually completed more than 390, including 14 Sea Tutors with long single-step Alclad floats. The structure of the Sea Tutor was specially treated against corrosion by sea water; a metal propeller and hand-turning gear were also standard. Because the latter features were desirable for a seaplane but not a landplane, the Sea Tutor was one of the few types of seaplane that could not be converted into landplane form. Power plant was a 160kW Lynx engine. In addition, numbers of Tutors were delivered to Denmark, Greece, Poland and South Africa, plus others for civil use. A further 57 were built under licence in South Africa for the air force and a very small number in Denmark for the navy.