Boulogne-Billancourt (bōlôˈnyə-bēyäNkōrˈ) [key], city (1990 pop. 101,971), Hauts-de-Seine dept., N central France, a suburb SW of Paris. One of the largest automobile factories in France is in the city. Other manufactures include airplanes, electrical goods, chemicals, bicycles, and processed foods. Part of the city is residential, with elegant homes bordering on the Bois de Boulogne. There is a 14th-century Gothic cathedral.