The C-3605 was a turboprop conversion of the C-3603 fighter-bomber of which 144 had been built between 1942 and 1944. The conversion prototype flew for the first time on August 19, 1968 and in total 23 C-3605 conversions were completed by late 1972.

The conversion needed a 7 ft (2.13 m) extention to the nose to balance the aircraft as the 555 lb (252 kg) mass of the 1,100 shp Lycoming turboprop was about half the weight of the original 1,020 hp Hispano-Suiza 12-cylinder V-engine.

EFW was the common used abbreviation for Eidgenössisches Flugzeugwerk (Federal Aircraft Factory). In 1972 EFW was renamed F + W, hence the type is also known as the F + W C-3605.