After designing the S.38 four-engined patrol flying-boat to Specification R.5/39, Saro built, as a private venture, a half-scale model as the Saro A.37, often called the Shrimp. The R.5/39 programme did not go ahead, but the Saro A.37 was completed in time to make its first flight in October 1939. Powered by four 63kW Pobjoy Niagara III engines, it was later modified to have a single fin and rudder, and other features representative of the Shetland flying-boat jointly developed by Shorts and Saro. In this guise, it acquired the serial TK580, and was tested from early 1944 until after the war had ended.