The D.IX was the last model to emerge from the D.VII concept.

The biggest change from the D.VII was the tail section of the D.IX and the engine choice.
The D.IX could also be equipped with an extra fuel tank between the wheels of the landing gear.
It was a single biplane with a 300 hp water-cooled Hispano-Suiza 8Fb V-8 engine.
The only built copy of the D.IX made its maiden flight in 1921.

The D.IX was sold to the U.S. Army Air Service in 1922 and shipped to the U.S. research center at McCook field.
In the USAAS, the D.IX received the designation PW-6, the sixth fighter aircraft in a series with a water-cooled engine, Pursuit W.ater-cooled.
There has been experimentation at McCook field with other engines on the D.IX, including a 320 hp Hispano-Suiza, Type 42 built in the US.
There have also been minor changes to the landing gear at McCook field, because both with and without an extra fuel tank has flown between the landing gear.
The D.IX could be equipped with synchronized armament, just like the D.VII, but for ammunition of a different caliber.

Despite the good climbing performance, the aircraft was not further developed for the USAAS, and it remained with the construction of one aircraft.