Derived from a 1917 project designated LeO 3, the LeO 9 single-seat cantilever monoplane fighter was of metal construction and of advanced aerodynamic concept. Powered by a 300hp Hispano-Suiza 8Fb eight-cylinder water-cooled engine, the LeO 9 was exhibited, prior to first flight at the Paris Salon in 1921. Flight testing was initiated in the following year, but, on 24 September 1923, the wing folded during a very sharp turn and the aircraft crashed at Villacoublay with the death of the pilot, Gaston Martin. Further development of the LeO 9 was then discontinued.