The fuselage of the D.I was rectangular-shaped, composed of a single longeron set at each corner and spaced via bulkheads in the forward section of the fuselage, which provided a mounting point for the engine bearers. Additional reinforcement was provided in the form of tubular steel diagonal struts that ran along the sides of the fuselage between the bearer and the lower wing root. Aft of the cockpit, the bulkheads were dispensed with for lighter frames composed of spruce, which were strengthened by diagonal struts; no internal wire bracing was used throughout the structure, relying upon the external plywood panels for rigidity.