The Saro A.21 Windhover was a British amphibious aircraft from the period between World War I and World War II, constructed by Saunders-Roe, or Saro.

It was originally advertised as the A.19 Thermopylae after the famous clipper ship, being an enlarged version of the Saro Cutty Sark.

[1] When tests to improve the power of the Saro A.17 Cutty Sark by adding a third de Havilland Gipsy II engine proved impractical (due to the additional weight on the small airframe), Saro designed a larger aircraft on similar lines that could indeed carry three Gipsy II engines.

Although a technically successful aircraft and nearly viceless in service, it was a type with a very limited market and only two were built.

[1] Data from Saunders and Saro Aircraft Since 1917[1]General characteristics Performance