Béchereau's subsequent wartime designs followed the basic outline of the SPAD S.VII. The two-seaters, the SPAD XI and SPAD XVI, were built in moderate numbers, around 1,000 of each type, but two-seater SPADs were much less successful than the rival Breguet 14 (5,500 built) and Salmson 2 (3,200 built). Single-seat developments of the SPAD VII were more successful. The SPAD XII was a minor variant, the first to use the geared Hispano-Suiza V-8 engine, which allowed it to be armed with a single-shot Hotchkiss cannon ("moteur-canon") firing through the propeller hub. Tested successfully by ace Georges Guynemer, the conclusion was that only very skilled pilots could exploit its powerful armament. Accordingly, although 300 were ordered, most were completed as normal SPAD fighters, with one (flown by Charles J. Biddle while with the USAAS' 13th Aero Squadron) and two may have served with the US Air Service in France.