The M.39 was designed by the Italian
aeronautical engineer Mario Castoldi to represent Italy in the 1926
Schneider Trophy competition. It represented a considerable departure from his prior designs, being the first low-wing
monoplane that Castoldi would design for Macchi, virtually nothing of the company's preceding racing flying boat, the
Macchi M.33, was present in its design. It was a single-seat twin-float seaplane of mixed (metal and wooden) construction. The pilot sat in an open
cockpit above the
trailing edge of the wing; the cockpits windscreen was profiled into the
fuselage decking to reduce
drag. This fuselage was
streamlined and intentionally avoided angularity. The adoption of such a radiator arrangement, which avoided the drag that would have otherwise been generated through the use of traditional
honeycomb or fin-type radiators, was considered to be a key innovation of the aircraft. The oil radiator was incorporated in the bottom of the fuselage in a relatively exposed position to achieve effective cooling. The M.39 was powered by a
Fiat AS.2 water-cooled V-12 piston engine, which was directly drove the
propeller. This engine, which was installed within the aircraft's nose, was equipped with three double
carburetors, one for every four cylinders, that were specifically designed as to enable them to operate normally at practically any orientation of the aircraft. The power-to-weight ratio achieved by this engine, inclusive of the water in the cylinder jackets and tubing, the residual oil, the starter apparatus and the
fuel pump, was exceptionally low (467 grams (1.03 lb.) per hp); it was reportedly lighter that any other powerplant in its power range in production with any other manufacturer in the world at that time. The M.39 was specifically built to compete for the Schneider Trophy; accordingly, its design had specialised features to aid it in this exact purpose. As the course circuit required left turns, the left wing of the aircraft had a slightly greater
span than the right wing, which permitted the aircraft to make tighter left-hand turns. To counteract
propeller torque reaction, the floats had unequal
buoyancy. and was powered by a 447-kilowatt (600-
horsepower)
Fiat AS.2 liquid-cooled
V12 engine, while the racing version had a 597-kilowatt (800-horsepower) Fiat AS.2. Macchi built two trainers, three racers, and one non-flying static-test
airframe. The first M.39, a trainer with serial number MM.72, was built in only a few months. It was soon followed by the second trainer (MM.73), the three racers (MM.74, MM.75, and MM.76), and the static-test airframe. ==Operational history==