The Junkers D.I (factory designation J 9) was a fighter aircraft produced in Germany late in World War I, significant for becoming the first all-metal fighter to enter service.What really set the Junker D.I apart from any previous aircraft was it's cantilevered low-wing design and corrugated duralumin skin. Duralumin, the same metal used for Zeppelin construction, was light yet strong. The Junkers monoplane was rigid, fast, and agile. The D.I was every fighter pilots' dream. The design was a decade ahead of its time.The prototype, a private venture by Junkers designated the J 7, first flew on 17 September 1917. Demonstrated to the Idflieg early the following year, it proved impressive enough to result in an order for three additional aircraft for trials. However, the changes made by Junkers were significant enough for the firm to redesignate the next example the J 9, which was supplied to the Idflieg instead of the three J 7s ordered.During tests, the J 9 was felt to lack the manoeuvrability necessary for a front-line fighter, but was judged fit for a naval fighter, and a batch of 12 was ordered. These were to have been supplied to a naval unit by September 1918, but instead equipped the same unit redeployed to the Eastern Front after the Armistice. One survives in a French Museum.