The gun chosen for the SPAD XII was not the old Hotchkiss cannon but a new 37 mm Semi Automatique Moteur Canon (SAMC), built by Puteaux, for which 12 shots were carried.

The SPAD XII also carried a single 0.303 inch synchronized (7.7 mm) Vickers machine gun mounted on the starboard side of the nose.

To accommodate the required geared output propshaft engine with a hollow propeller shaft for the cannon to fire through, and power the airframe, weighing 587 kilograms (1,294 lb) rather than the 500 kilograms (1,100 lb) of the SPAD VII, the 180 bhp Hispano-Suiza 8 direct-drive Ab engine was replaced by the geared 220 bhp model 8Cb, which also gave the SPAD XII a clockwise (seen "nose-on") rotating propeller.

Test-flown by Guynemer, the early production models of the SPAD XII were highly successful after overcoming initial problems with the reduction gear between engine and propeller.

Single examples for testing were delivered to the Royal Flying Corps[5] and one to the Aviation Section of the American Expeditionary Force, with the AEF's 13th Aero Squadron receiving the code "0", it was primarily flown by the 13th's Charles John Biddle.