The American development of antiaircraft artillery had, previously to 1917, been confined almost exclusively to the task of designing and constructing stationary units of defense for our coast fortifications. It was naturally expected that it would be at those points that we would first, if ever, have to meet an attack from the air. Very little attention had been paid mobile artillery of this sort. Before April, 1916, the Ordnance Department had designed a high-powered 3-inch antiaircraft mount for the fixed emplacement at coast fortifications. The gun on this mount fired a 15-pound projectile with a muzzle velocity of 2,600 feet a second. It is still to-day the most powerful antiaircraft weapon of its caliber. Between May, 1916, and June 18, 1917, orders for 160 of these mounts were placed with the Watertown Arsenal and the Bethlehem Steel Co. Up to April 10, 1919, a total of 116 of these had been completed and sent for emplacement at the points selected. [Illustration: TWO VIEWS OF ANTIAIRCRAFT GUNS ON TRUCK MOUNTS.] [Illustration: TWO OTHER VIEWS OF ANTIAIRCRAFT GUNS.] By the end of 1916, however, it was foreseen that it would be necessary to provide antiaircraft artillery of a mobile type as part of the equipment for any field forces that might be sent abroad. Since that contingency seemed entirely possible at that time, and as it appeared to be impossible to provide a suitable design that would have a sufficient period of time in which to get proper consideration and test, it was decided to improvise a simple structural steel design that would permit quick construction and on which a 75-millimeter field gun, that was already in production, could be mounted. This design was completed May 1, 1917, and an order for 50 placed with the Builders Iron Foundry. Deliveries on these were made during the fall of 1917, and the carriages were at once shipped to France for equipment with French field guns and recuperators that had been already procured for the purpose. In its mobility the improvised antiaircraft gun mount was far from perfect. It was necessary to disassemble it partly and mount it on trailers. The need for a mount that could be moved easily and speedily had been realized before our entrance in the war, and a design embodying these qualities was completed as early as December, 1916. This truck was designed to be equipped with the American 75-millimeter field gun, model of 1916. Before the drawings were completed an order for the pilot mounts of this type was placed with the Rock Island Arsenal. The war came on, and it was decided not to wait for a test of the mounts before starting general manufacture. Accordingly the New Britain Machine Co., in July, 1917, was given an order for 51 carriages. No further orders were placed for carriages of this sort, as it was not thought best to go too heavily into production of an untried mount. It may be noted here that our first 26 antiaircraft guns were mounted on White 1½-ton trucks.