With the arrival of World War 2 in September 1939, many existing aircraft of all types were pressed into wartime service and the resulting actions often showcased obsolete approaches to both design and doctrine. Additionally, aviation technology began a quickening pace of evolution which rendered many prewar mounts expendable or useless in the coming years. Such was the case with the Italian Air Force's (Regia Aeronautica) Savoia-Marchetti SM.79 "Sparrowhawk" tri-engine bomber series. It was introduced in 1936 and some 1,350 were produced from then until the end of the war in 1945. However, by 1940, a newer, more modernized bomber form was sought.
Savoia-Marchetti engineers took the basic three-engine design with its low-set monoplane wings and modernized it in the hopes of a better product. The fuselage was revised and uprated engines installed. The original wings were retained and the cockpit flightdeck remained in stepped position over the long nose assembly. The tail unit consisted of a dual-vertical tail fin arrangement and the undercarriage was of a "tail-dragger" configuration. Armor protection for the crew and systems were improved over that of the "softer" SM.79 series and self-sealing fuel tanks were used. Construction included a steel tube understructure with metal skin and fabric at the fuselage while the wings were of wood. The fuselage housed an internal bomb bay.
A prototype was developed and this recorded its first flight on June 5th, 1940. After the requisite flight testing phase, the aircraft was ordered into production by December and formally introduced during 1941 as the "SM.84". It was intended to succeed the aging SM.79 series in full.
The three-engine concept allowed for additional pulling power without a major disruption of the overall design. Several well-known tri-engined aircraft were used throughout the war including Ford's famous Tri-Motor as well as the recognizable contribution by Junkers with its Ju 52.
As completed, the SM.84 featured a crew of up to six - pilots, radioman, engineer, bombardier, and machine gunners. Dimensions included a length of 18 meters with a wingspan of 21 meters and height of 4.6 meters. Empty weight was listed at 19,500lbs with a loaded weight nearing 29,330lbs. Power was served through 3 x Piaggio P.XI RC 40 14-cylinder, air-cooled radial piston engines - one fitted at the nose and the other two along each wing leading edge as normal. Output was 1,000 horsepower each which allowed for a maximum speed of 270 miles per hour with a range out to 1,140 miles and a service ceiling of 25,920 feet.
Armament began with the standard installation of 4 x 12.7mm Scott heavy machine guns. These were positioned as defensive systems about the fuselage. One was added to a dorsal position over and aft of the cockpit with another manned at the belly (rear). Each beam area was also given a machine gun to protect the vulnerable sides of the aircraft. Frontal attacks should have to be countered by the dorsal gunner but the gun's position was such that it was restricted from firing through the nose-mounted engine's spinning propeller blades. In addition to her local defensive suite, the SM.84 was also given an internal bomb bay to house up to 4,400lbs of drop ordnance. Additionally, she was expected to handle anti-ship sorties as well and therefore cleared to carry at least two Italian Navy torpedoes in her belly in lieu of bombs.
With all that said, the SM.84 was initially deployed to 12th Squadron of the 41st Bomber Group on February 2nd, 1941. In practice, the bomber did not prove the successor to the SM.79 originally envisioned. Surely there were some early, albeit limited, successes but these were offset by losses in turn. There was a successful torpedo attack against HMS Nelson which knocked her out of the war for a time but this was against a loss of six Italian bombers and crew. In fact, much of her service career saw the SM.84 used as a maritime patrol and strike platform and only one torpedo was typically carried in her hold. Operations involved the aircraft over Malta and during the Allied "Operation Torch" landings in North Africa.
By the time of the Italian surrender in September 1943, just 150 SM.84s were available and, of these, only about 100 remained in flyable condition. As proved the case with other Regia Aeronautica aircraft following the surrender, the SM.84 fell into the inventories of both the pro-Allies "co-belligerent" Italian air service and the Axis-aligned Aeronautica Nazionale Repubblicana but these proved few in number and made little impact in ongoing actions - the preference was to field fighters instead. The German Luftwaffe claimed a few examples of their own and some were delivered to Slovakia to support the Axis cause before the fall of Italy en route to Berlin.
While managing a career into the final days of World War 2, the aircraft was quickly retired from frontline service in the period following. It was not a popular design with crews nor commanders and never fully replaced SM.79s in service. Only one variant of note emerged, this being the SM.84bis, which attempted to rectify some issues but was, itself, no more a success. The SM.84ter was a "one-off" development appearing in 1944 and fitting 3 x Piaggio P.XII series engines of 1,500 horsepower each. This speedier design was lost in a landing during 1946 and not furthered.
Sources list total production at 309 aircraft.