"13"^^ . . . "The Junkers G 31 was an advanced tri-motor airliner produced in small numbers in Germany in the 1920s. Like other Junkers types, it was an all-metal, low-wing cantilever monoplane. Compared to modern aircraft, only the corrugated duralumin metal skin, so typical for Junkers aircraft, seems unusual. However in the mid-1920s all-metal construction and an aerodynamically 'clean' configuration were remarkable."@en . .