The Junkers J.I (manufacturer's name 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. Design 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 as armour, 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 thick and weighed and protected the crew, the engine, the fuel tanks and radio equipment. 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. There was a significant size difference between the upper and lower wings – the upper wing had an area of , over double the area of the lower wing – . This is a form of biplane known as a Sesquiplane. The aircraft had two fuel tanks with a capacity of around . The main tank (divided in two for redundancy) was supplemented by a smaller, gravity tank. This was intended to supply fuel to the engine by gravity feed in the event of an engine fuel pump failure; it contained enough fuel for thirty minutes on full power. There was a manual fuel pump for use when the gravity tank was empty. The aircraft could be separated into its main components: wings, fuselage, undercarriage and tail, to make it easier to transport by rail or road. A ground crew of six to eight could reassemble the aircraft and have it ready for flight within four to six hours. The wings were covered with aluminium skin which could be easily dented; great care had to be taken when handling the aircraft on the ground. Operational history The J.I was well liked by its crews, although its ponderous handling earned it the nickname "furniture van". The aircraft first entered front service in August 1917. They were used on the Western Front during the German spring offensive of 1918. The aircraft could be fitted with