The Junkers J 1, nicknamed the Blechesel (Tin Donkey or Sheet Metal Donkey), was an experimental monoplane aircraft developed by Junkers. It was the first all-metal aircraft in the world. Manufactured early in the First World War, an era in which aircraft designers relied largely on fabric-covered wooden structures braced with wires, the J 1 was a revolutionary development in aircraft design, making extensive use of metal in its structure and in its outer surface.

The J 1 originated from the work of pioneering aeronautical designer Hugo Junkers. The experimental aircraft never received an "A" or "E-series" monoplane designation from IdFlieg, the Army inspectorate of military aircraft and The Imperial German Air Service (Die Fliegertruppen des deutschen Kaiserreiches). The aircraft was known only by its Junkers factory model number of J 1 and should not be confused with the later, armoured all-metal Junkers J 4 sesquiplane, accepted by the later Luftstreitkräfte as the Junkers J.I (using a Roman numeral), from the category of armored combat aircraft established by IdFlieg.

On 12 December 1915, the aircraft made its brief maiden flight, flown by Leutnant Theodor Mallinckrodt of Flieger-Ersatz-Abteilung 1 (FEA 1), during which an altitude of almost 3 m (9.8 ft) was reached. Greater altitudes and performance were achieved during subsequent flights. By the end of January 1916, Junkers had been given a contract to further develop his all-metal concept and the later Junkers J 2 single-seat fighter, which would never see front line service, followed the J 1. It is believed that the Junkers J 1 was not flown again after January 1916. In 1926, it was placed on static display at the Deutsches Museum in Munich. In December 1944, the J 1 was destroyed during an Allied bombing raid on the city.

Amongst the earlier pioneers and innovators in the field of aviation was the German engineer and aeronautical designer Hugo Junkers. During his early career he had established his engineering credentials outside of the field of aviation; Junker's innovations had included the invention of a type of calorimeter and in the construction of internal combustion engines. Sometime after 1897, Junkers was first introduced to the principles and field of aviation after having received details of the concept of heavier-than-air travel from Hans Reissner, a colleague and fellow professor at the Technische Hochschule in Aachen, where Junkers held the chair of Professor of Thermodynamics.

In 1907 Reissner approached Junkers, seeking his collaboration in the design and construction of an early monoplane aircraft; although this first effort, which flew in 1909, did not meet with much success, it was this project which has been credited with leading to Junkers pursuing a career as an aeronautical designer. Five years later Reissner, with Junkers' help, began construction of his all-metal canard design, which he named the Ente (Duck) which first flew on 7 August 1912. Junkers' firm manufactured portions of Reissner's design, including the flying surfaces and radiator. The problems encountered in constructing the Ente had led to Junkers spending considerable amounts of time working on the problems of airframe design, including an examination of the options for the elimination of the practice of exterior bracing of airframes. During 1910 he patented the design for a fully cantilevered thick aerofoil tailless aircraft (now known as a flying wing) in Germany.

During 1910 Junkers received a grant for the construction of a wind tunnel at his research facilities in Aachen; this led to the initiation of an aerodynamic research programme that would, five years later, contribute to the design of the Junkers J 1. In 1911, Junkers resigned his professorship to dedicate his efforts to his Dessau-based engine company; he returned to Aachen upon the completion of the wind tunnel. It was not until 1915 that Junkers was able to fully devote his time to the design and manufacture of an aircraft; it was at this point that he opened a research institute, the Forschungsanstalt Professor Junkers, which was assigned responsibility for the design and development of a series of fully cantilevered all-metal monoplanes.

Upon the outbreak of the First World War in August 1914, Junkers decided to direct his efforts towards projects which would have potential military value. The majority of aircraft designers were relatively conservative and, save for some isolated examples, no advances were achieved; according to aviation historian Charles Gibbs-Smith, the pioneering work of Hugo Junkers was a notable exception. It was Junkers' efforts, along with those of collaborators such as engineers Otto Reuter, Otto Mader, head of the Forschungsanstalt and Hans Steudel, director of Junkers' structural materials and testing department, that the J 1 would be produced as a private venture, in the form of what would now be termed a technology demonstrator.

Junkers and the Forschungsanstalt, commenced engineering work to realize his concept for the creation of aircraft designs that would dispense with drag-producing exterior bracing. His work on Reissner's Ente design had convinced him of the necessity to use metal as the main structural material. Although duralumin, which had been invented by Alfred Wilm six years earlier, was apparently the ideal metal alloy for aircraft construction it was prone to flaking and other undesirable characteristics when worked in sheet metal form. The early all-metal aircraft designs produced by Junkers used sheets of heavier electrical steel, similar to the types of ferrous sheet metals that are typically used in laminated-core AC electrical transformers.^{[citation needed]}