I've found the item (source: 'Aerei nella Storia' magazine, issue #33 Jan 2004, West Ward editions, author Daniele Lembo,
WWW.AEREINELLASTORIA.IT) and here is a summarized translation up to Spanish war, let me know if you are interested in WW2 service and I'll translate it.
It is very detailed on the aircraft and the WW2 service, unfortunately not so much detailed on Spanish war operations.
There is also plenty of photos and nice drawings, but unfortunately I can't scan and post them for a while (i am leaving and will be back home on oct 13): let me know if you are still interested by then.
-Origin
On July 4th 1934 the Savoia-Marchetti SM73 fast liner took the air. Was a 3 engine cantilever monoplane, one of the first land based liners to challenge the supremacy of seaplanes for long range passenger Transport. In 1935 the plane was used for air connection between Rome and Asmara (6000km) by Italian companies and between Bruxelles and
Elizabethville (Congo) by the Belgian SABENA. Seating was for 18 passengers and 4 crew in a (by then) luxury cabin.
Apparently the SM73 was originated by a request from Italo Balbo to 'put a simple fuselage on the good wing of the S55 seaplane' in order to have a modern transport for the colonial empire.
In parallel the military bomber/transport version was developed: the SM81 whose maiden flight was Feb. 8,1935.
-Construction
The fuselage was a frame of steel tubes, covered in fabric and plywood,except for the cockpit area that was covered in alloy sheet.
Internal illumination was provided by 8 windows per side plus 3 on the fuselage top; two further openings toward the rear side were available for the defensive MG
The cantilever wing had 3 'double T' shaped wood spars and was skinned in plywood. Rudder and tail were in steel tubes covered in fabric. Landing gear was fixed, with air operated brakes.
The emergency exit was trough the upper glass panels of the cockpit
A narrow passage enabled the inspection of the wing engines
The bombsight was fitted in a retractable cabin on the cockpit floor, to be lowered during the bomb run. The cabin was also jettisonable for emergency exit.
Bomb bay was in the fuselage center, two rows with a central passage.
- Engines
3 , one in the nose and one per wing, 3 bladed metal propellers, diameter of 3.40-3.50 mt
several engine types were installed, in order to standardize the logistic per operating theatre with other aircrafts.
Alfa Romeo 125 RC 35, 650 Hp
Gnome&Rhone K14RC, 740 hp
Piaggio PX RC15, 700 hp
Alfa Romeo 126 RC34, 750 hp
Piaggio PIX RC40, 780 hp
the Alfa Romeo were used in Europe, the Piaggio in East Africa and the G&R in Lybia/Egypt
The engine cowlings were of course different in shape, depending on the fitted engine
Fuel capacity was 3620 or 4400 (G&R engines) liters, divided in 2 fuselage and 6 wing protected tanks
-Dimensions
Wingspan : 24 m
Area : 92 sqm
Lenght : 17.9 to 18.30 m, depending on engines
Height : 4.30
Empty weight : 6750 to 7300 kg, depending on engines
MTOW : 10505 kg
- Performances
top speed : 325 to 350 kmh depending on engines
Cruise : 260 kmh
operative ceiling : 7000 m
range : 640 km full loaded, 1500 km 'light', 2000 km max
take off run : 230 to 290 mt
landing run : 200 m
- crew : 6 (pilot, co-pilot, radio operator/waist gunner, bomber/waist gunner, 2 gunners)
- special Versions
VIP transport (was one of the planes assigned to Benito, who used to fly it personally)
Air ambulance (5 units)
Air tanks
Radio transmission
Minesweeper (magnetic ring)
- Armament
6x 7.7 mm machine guns, two manually handled Lewis for the side and 4 SAFAT in 2 twin hydraulically operated turrets (upper top and lower bottom fuselage)
Bomb load : 2000 kg, in various bomb combinations.
Bigger bombs (250 and 500 kg) fit horizontally in the bomb bay, smaller bombs were installed vertically.
Possibility to carry 2x800 kg torpedo (but operative trials were not successful)
- Operativity
Delivery started in 1935, to
7th and 13th Stormo, Lonate Pozzolo (where now is Malpensa intl airport)
9th Stormo, Ciampino (Rome)
15th Stormo, Ferrara
First operation was in Ethiopia (Piaggio engines) : bombing, reconnaissance and air-drop supply. 36 units remained there after the war.
In 1936 was delivered to
8th Stormo, Bologna
10th Stormo, Bresso (Milan)
11th stormo ,Ferrara
12th stormo ,Guidonia (Rome)
14th and 16th Stormo, Vicenza
32th and 33rd, Naples
In 1936 the SM81 was posted in Lybia (G-et-R engines)
In July 1936, 12 SM81 (Alfa Romeo) flying from Sardinia joined the Nationalist party in Spanish war.
Officially the airplanes were 'purchased' by a Spanish citizen, the crews were not officilly Italian soldiers.
Due to bad weather only 9 arrived in spain.
The small force was first used to bomb the Republican Navy (August) in order to protect the troop transport of Franco, then were redeployed to day and night bombing.
Was here that the night bombers were named 'Pipistrello' (bat), and the name remained for good.
After the war, the remaining SM81 were kept by the Spanish air force.