A turbine powered Otter had a long gestation. DHC considered it after the P & W Canada PT-6 engine was developed in the '60s (too expensive and the DHC-6 Twin Otter was preferred). Western Rotorcraft in Seattle, WA mounted a Garrett TPE311 to N3904, but the improvement was unremarkable and the piston engine was re-installed. Ray Cox engineered his conversion in 1973 and tested C-FMES-X / N4247A into the '80s. An unfortunate forced landing on December 19, 1984 damaged the prototype and funds ran out before the program could continue with only one aircraft converted.

This 1961 Otter was acquired by Ray Cox from an insurance company in 1974 after it was damaged in a take off accident when the engine failed. Flying as Turbo Otter C-FMES-X with his PT-6 powered Cox Conversion in 1978, the testing moved to Renton, WA (later Boeing Field) in 1981 for more development funding.