The prototype had a four bladed wooden propeller on the central engine, and two bladed wooden propellers on each wing engines, later all aircraft were fitted with 3 bladed metal propellers and was powered initially by three Mistral Kfr engine
Otherwise it was similar to the S.81 Pipistrello, but it was never completely clear if it was a derivative or a parallel development of the S.73. In any case, S.73 was very similar to the S.81, being a mixed construction (a skeleton of steel covered by wood and fabric for the fuselage, wood for the 3 spar wing) monoplane with a braced tailplane and fixed undercarriage. It had 3 engines, always of the same type and model. Electro-generators were two, in the sides of fuselage, the batteries had 24V-90A.
The pilot and co-pilot were seated side-by side in an enclosed cockpit, with a compartment for a radio operator and a mechanic. A passenger compartment could house 18 passengers in two rows.
It had 8 metallic fuel tanks, all in the wings, with a total capacity of 3,950l. The prototype had French engines, the Gnome et Rhône Mistral Kfr, but further aircraft had, depending by various reasons, 700 hp Piaggio P.X Stella, 770 hp Wright Cyclone R 1820, 730 hp Walter Pegasus III MR2V and Alfa Romeo AR 125 and 126. Propellers were three-blades, aluminium-steel variable pitch (only adjustable on the ground).
It could be used from small airports and had reliable handling controls. It was not too costly. The power on board was incremented with the latest types of engines. Piaggio P.X, 700 hp, Wright R.1820, 770 hp, Walter Pegasus, 730 hp, Alfa AR.125/126. With R.1820 had 2310 hp and a speed cruise/max 270/340, 1000 km range, 6.300m. ceiling. Interestingly, with the less powerful AR.126 it had 2250 hp, 345 km/h, 1000 km, 7,000 m ceiling, possibly because the best outpower at altitude. Finally, the SABCA license-produced aircraft had 900 hp Gnome-Rhône engines for a total of 2,700 hp, comparable to the last models of S.79s or the Cant Z.1018.
The military SM.81 variant served as a bomber, transport, and reconnaissance plane. It was operational in Abyssinia and Spain.
In December 1935, was used for a journey from Italy to Asmara, sending over 200,000 letters, with 6,600 km journey made in 4 days and then the return to Rome was on 6 January. A commercial line was established in a 6,100 km journey. SABENA had a similar line with Congo, with 4 days and 44 flying hours.
This aircraft was, from the start, a valid project, and few modifications were recommended by Regia Aereonautica. Also for the S.81. It was easy to fly, rugged, easy also to operate at ground, included the ability to fly from short airfields and with bad terrain conditions, in spite of its relatively underpowered propulsion system and the lack of slats. Its mixed construction and the fixed landing gear were the main shortcomings, when in USA and Germany there were already full metallic aircraft, that not always, like in the case of Ju-52, were also faster or with better performance, and surely costly compared to this machine, that was competitive with them for some year.
When war broke this aircraft was obsolete, but was included in transport units, almost non existent before the war, with 8 S.73 militarized in Italy and used for 605° Squadriglia cargos. Despite a long and dangerous employ, at least two survived until the Armistice and then used by 103° of RSI Aviation.
Five S.73s were present in Eastern Africa and used as well as military transports. Of these latter, one escaped by Ethiopia, when it fell into British hands. The former 5 S.73s were I-GELA, I-NOLA, I-NOVI, I-ARCO and I-VADO. I-GELA was destroyed by bombers, I-NOLA was destroyed by a flight incident. But their most difficult task was the final one.
The others three were sent by the Duke of Aosta, local commander in Italy, seen the fall of the military situation. After several days of work the three aircraft took off from Addis Ababa, with 36 men inside and an overload of fuel, 3 April 1941. They had the mission to reach Cufra, 2,500 km away. The S.73 had a normal range of 1,000 km with 1,500 payload, so much more fuel was added to normal tanks, with many tanks in the fuselage. All the three aircraft force-landed in the desert. Luckily they were able to took off again, and reached Jeddah, where made an intermediate refuelling.
After several days of inenarrable difficulties to keep efficient the aircraft, included sand storms that filled all the air filters, they took off again. Initially it was planned to make another landing to Beirut but in the mean while Rommel had conquered Benghazi, so this was the final destination of the three aircraft, one of them led by Max Peroli. Two aircraft, after 10 hours of fly, with men inside almost killed by fumes of the fuel auxiliary tanks (and with absolute forbid to smoke), finally landed at Benghazi, after 4,500 km and over a month. It was 8 May, and only two aircraft managed to do so, while I-VADO landed after several days, because the damages were too heavy. So the three logored S.73s (the original reason of the forced landing over the desert was the leaking of the eroded fuel tanks) managed to reach homeland, with over one month and many risks, included the arrest in Arabia, for which English pushed, until Italians managed to took off.
Seven Belgian S.73s were flown to the United Kingdom in May 1940 and were impressed into service by the Royal Air Force. The RAF examples were used in North Africa where four where later used by the Regia Aeronautica. Some Italian aircraft were impressed into military service in East Africa while those still in Italy were used to equip 605 and 606 Squadriglie. Four S.73s had survived by the 1943 armistice, three being used by the allies and with one by the pro-axis government, but all had been taken out of service by the end of the war.