Forerunner of the SM.81 bomber that fought in Ethiopia, Spain, and in the Second World War, the SM.73 three-engined transport was produced in 1934. The prototype was powered by 600 h.p. Gnome-Rhone 9 Kfs radial engines driving wooden airscrews (four-bladed on the nose engine, two-bladed on the outboard units), and featured the tall tail surfaces of the SM.71. The SM.73 went into production with a revised, longer fuselage, 700 h.p. Piaggio Stella IX R.e. nine-cylinder radials driving three-blade variable-pitch metal airscrews, and a lower, broader vertical tail assembly. As the SM.73P it entered service with Ala Littoria; as well as S.A.B.E.N.A., the Belgian airline; and C.S.A., the Czechoslovakian state airline.
The SM.73, weighing 12,760 lb. empty and 20,460 lb. loaded, had an all-wood cantilever wing and fabric-covered welded steel tube fuselage and tail group. Wheel spats were filled, open on early models, later fully-enclosing. Eighteen passengers and a crew of four were accommodated, plus up to 800 lb. of baggage in the bottom of the fuselage. Maximum speed was 205 m.p.h. at 13,120 fl., cruising speed 174 m.p.h., and landing speed 56 m.p.h. Service ceiling was 24,272 fl. and maximum range 994 miles. The SM.73 climbed to 6560 ft. in 10 min., to 13,120 ft. in 20 min., and to 19,680 ft. in 33 min. On two engines, maximum speed was 168 m.p.h. and service ceiling 14,432 ft. Dimensions were: span 78 ft. 8 2/3 in., length 57 ft. 2 3/4 in., height 15 ft. 1 in., and wing area 1000.7 sq. ft.
With Italy's entry into the war, the SM.73's of Ala Lilloria came under the management of the Commando Servizi Aerei Speciali (C.S.A.S.) as military transports. Thirteen formed the 605a and 606a Squadriglie of the 1480 Gruppo "T", based at Reggio Calabria. These were used for varied duties, including the supply of troops and equipment to North Africa. In many ways the Italian counterpart of the German Junkers Ju 52/3m, the SM.13 was one of the earliest transports to serve efficiently on European air routes. A few were built by S.A.B.C.A. in Belgium.