Late in 1917 new, more powerful rotary engines were appearing and the Idflieg encouraged aircraft manufacturers to incorporate them into fighters.

The upper plane was a little larger in both span and chord, so the two interplane struts on each side diverged slightly upwards and leant gently outwards.

The rotary engine drove a four blade propeller, an unusual feature at this time and only fitted to one other LFG fighter, the later D.XVI.

The D.IX's conventional undercarriage was of the single axle type, mounted on V-struts to the lower fuselage and with a tailskid attached to the ventral fin.

[2] The first prototype flew in the first D type competition, held in January 1918, and performed well but was lost soon after in an accident in which the pilot's seat collapsed, followed by an inadvertent loop with g-forces that expelled him through the fuselage bottom.