Yes, flamethrowers were used in World War I. Although they were not developed until the end of the 19th century, they quickly proved to be devastatingly effective weapons in trench warfare. Flamethrowers were first used in the war by the German Army in 1915 at Givenchy-les-La Bassée. The French also quickly adopted flamethrowers and the American Expeditionary Force used them in the last year of the war.