In 1940, the Regia Aeronautica received a new addition to its bomber force, the SM.84, that was designed to replace the SM.79, but which never succeeded in equaling the performance of its illustrious predecessor. This was principally due to its lack of maneuverability, making it anything but ideal in the role of torpedo-bomber, and due to the general lack of reliability of its Piaggio P.XI engines.
The project was launched in 1939 and the first flight of the prototype took place on 6/5/40. Marchetti based his proposal on the SM.79 airframe, using the same wings and modifying the fuselage and empennage. The fuselage lacked the characteristic humpback appearance of the SM.79 while the empennage (tail section) were doubled. The SM.84 was a low-wing monoplane made of wood and steel tubing and covered with fabric, plywood and aluminum. The SM.84 was powered by three 1,000-hp Piaggio P.XI radial engines driving three-bladed variable-pitch metal propellers. Defensive armament consisted of four 12.7mm machine guns in the back, belly and 2 on the sides of the aircraft. The bomb load could be housed either in the belly (2,207-lb) or under wing (2 torpedoes, rockets or 3,532 lbs of bombs). Top speed was 268 mph at 15,000 ft with a service ceiling of 25,900 ft and a range of 1,137 miles.
Prior to the prototype’s maiden flight, several tests were carried out with a specially modified SM.79 provided with double empennage and 860 hp Alfa Romeo engines, and the aircraft’s performance and potential had proved to be generally satisfactory. This was not so when the SM.84 prototype began its evaluation tests. It immediately proved to have a series of problems, especially at takeoff and landing, principally caused by the great weight of the wings and by the inadequacy of the vertical empennage. Moreover, the Piaggio engines proved to be unreliable and difficult to build. Despite these problems, a large number of SM.84s were ordered by the Regia Aeronautica with an initial request for 246 aircraft placed at the same time that the prototype and evaluation aircraft appeared. Eventually orders amounted to 309 aircraft.
The SM.84 began its operational career with the 41st Bomber Group in 2/41 and several months later it went into service with the 36th Stormo Aerosiluranti. The SM.84 served in the torpedo-bomber role for about a year, until autumn of 1942, when it was reassigned to bomber units.
In the meantime, attempts to improve the SM.84’s performance led to the SM. 84bis with modifications to the wings and cockpit as well as ventilation to the engines and a better torpedo launching control. However, the SM.84bis went into service with bomber units, where they operated until the armistice. In 7/43, 43rd Stormo was the only unit operating the SM.84 and by 9/8, had 30 aircraft at its disposal. A further 130 bombers, of which about 100 were effective, were distributed among several supply centers for the Regia Aeronautica. Following the armistice, the Germans incorporated 12 or so SM.84s into the 132nd Transport Group where they served until the end of the war.