The history of Junkers aircraft production begins with the Junkers J 1 mid-wing monoplane (not to be confused with the later, all-metal sesquiplane ground attack aircraft with the "IdFlieg" designation J.I which had a factory designation of J 4). Research for this aircraft began in 1914 and was interrupted by the start of the First World War. The prototype aircraft, named the "Blechesel" (Tin or Sheet-metal Donkey), was completed in very late 1915 after the outbreak of the war. This aircraft is significant in that it was the first flyable aircraft to utilize an all-metal "total structural" design. Contemporary aircraft were built around wooden frames constructed in a rib-and-stringer fashion, reinforced with wires, and covered with a stretched fabric. The J 1 was a semi-monocoque design, using steel ribs and sheeting that formed both the stringers and the skin. At the time aluminium was still fairly expensive; and the desirable 1906 invention of German metallurgist Alfred Wilm, that of duralumin alloy, could not be worked in sheet form without damaging flaking occurring, so the J 1 was made of sheet electrical steel. It was quite heavy as a result, which translated into poor climb and maneuverability, yet its clean monoplane layout, which even featured a "belly" radiator installation for its Mercedes D.II inline-six cylinder engine, had very low drag, and the J 1 was one of the fastest aircraft of its day, reaching speeds of 170 km/h, with only a 120 hp engine for power.