The ultimate development of the basic D VII single-seat fighter biplane, the D IX was flown for the first time in 1921 powered by a 300hp Hispano-Suiza 8Fb eight-cylinder water-cooled engine. Of typical Fokker construction in employing a welded steel-tube fuselage and wooden wings each having two box spars, the D IX featured an aerofoil-shaped fuel tank between the main wheels as first tested by the V 36. The sole prototype of the DI X was purchased for evaluation by the US Army Air Service and shipped to the USA in 1922, being assigned the designation PW-6. As evaluated by the USAAS, the aerofoil-section fuel tank was deleted, but the PW-6 was not considered to offer a sufficient advance over the D VII to warrant further development.