Following the J 1, a series of "J-designated" (by the Junkers firm itself) aircraft followed, each advancing the state of the art in terms of strength and weight, but no single design progressed much beyond the prototype stage in terms of production potential. The Junkers J 2 was an extensively "cleaned up" and armed single-seat fighter descendent of the J 1, while the J 3, a never-completed single rotary engined mid-wing monoplane design that only consisted of covered wing panels and a nearly complete fuselage frame, replaced the smooth exterior electrical steel sheeting with duralumin. "IdFlieg", in charge of aircraft evaluation, was unconvinced of the monoplane layout of these designs, and ordered a sesquiplane design as the J 4. Junkers took this opportunity to produce all the flying surfaces (wings and horizontal stabilizer) of the J 4 aircraft of corrugated duralumin (except for some fabric on the rear fuselage, and the armored steel fuselage "bathtub") in order to lower weight. The J 4 became Junkers' first design to enter production, with orders totalling 283, of which about 184 were delivered to operational units. Since it was the first design from Junkers to serve in the Luftstreitkräfte's ""J"-class" of armored, infantry co-operation aircraft, which also had the Albatros J.I and AEG J.I serving with it in the same capacity, the curious and confusing instance of the Junkers J 4 armored all-metal sesquiplane getting the German military designation "J.I" was one caused solely by the Luftstreitkräfte's choice of designation letter for "all" of its armored, ground forces co-operation aircraft.