The Junkers J.I (manufacturer's designation J 4) was a German "J-class" armored sesquiplane of World War I, developed for low-level ground attack, observation and Army cooperation. It is especially noteworthy as being the first all-metal aircraft to enter mass production; the aircraft's metal construction and heavy armour was an effective shield against small arms fire over the battlefield.
It was an extremely advanced design for the period, with a single-unit steel "bathtub" running from just behind the propeller to the rear crew position acting not only as armour, but also as both the main fuselage structure and engine mounting in one unit. Engine access was provided by armored steel panels, one on either side of the nose. The armour was 5 millimetres (0.20 in) thick and weighed 470 kilograms (1,040 lb). It protected the crew, the engine, the fuel tanks, and radio equipment, when fitted. The flight control surfaces were connected to the aircraft's controls by push-rods and bellcranks – not with the usual steel cable control connections of the era as push-rods were less likely to be severed by ground fire.