Despite its design, its development and ultimately its production at Caproni's subsidiary, the Ca.135 was originally to have been built at the company's mani facility, and it is this factor that explains why the type had a designation in the main Caproni sequence rather than in the Caproni Bergamasca Ca. 300 series.

The spurs to Caproni's initial commitment to the design in 1934 were the arrival from Breda of Cesare Pallavicino as chief designer, and the release of an Italian air force requirement for an advanced twin-engine medium bomber. This requirement called for a maximum speed of 240 mph at optimum altitude, the ability to carry a bomb load of 2,646 pounds over a range of 621 miles and the ability to maintain an altitude of 16,450 ft on the power of one engine. Pallavicino laid out a trim cantilever monoplane of mixed construction with the two engines located in wing-mounted nacelles and the crew grouped in the forward and central fuselage. The fuselage was based on a forward section of light alloy semi- monocoque construction to which was added a rear fuselage section of welded steel tube with fabric covering. To this were added the tailwheel landing gear (including a semi-retractable tailwheel and main units that retracted into the underside of the two engine nacelles) and the flying surfaces: the tail unit comprised a high-set tailplane with twin vertical surface located about mid-span of its two halves, and the wing comprised two dihedraled halves that were tapered in thickness and chord. The wings were of mixed metal and wood construction and had plywood skinning and fabric covering on their forward and rear sections respectively. Defensive armament was provided in the form of three Breda turrets: each could carry a single 12.7mm machine gun or two 7.7mm machine guns, and the dorsal and ventral turrets were of a semi-retractable type to reduce drag in cruising flight.

The prototype made its first flight in 4/35 powered by two 800 hp Isotta-Fraschini Asso XI RC Vee engines and driving a wooden two-bladed propeller. The propeller was soon exchanged for a three-bladed metal propeller subsequently accepted for all production versions. The Italian air force felt the Ca.135 had potential and in 1936 ordered 14 examples of the Ca.135 Tipo Spagna (Spanish model), presumably as it was intended to undertake an operational evaluation in the Spanish Civil War. However, none of the aircraft served with the Italian air force in Spain. The Ca.135 Tipo Spagna was then re- designated as the Ca.135/Asso for its two 836 hp Asso XI RC.40 Vee engines, but even with this slightly uprated powerplant, the performance of this pre-production model was degraded in comparison with the prototype. In addion to the operational equipment and extra fuel capacity, the weight was increased that resulted in a lowering of the maximum speed to 249 mph and the cruising speed to 227 mph at optimum altitude. The climb rate and service ceiling were also lowered while the range was increased.

The Ca.135 Tipo Peru (Peru model) was initially ordered in 1936 for 6 bombers but eventually 32 bombers were delivered. The aircraft differed from the initial Italian model in its revised gun positions and cleaner nacelles for the two 900 hp Asso XI RC.40 engines. These aircraft were used operationally by Peru in the Grand Chaco War of 1941 against Ecuador and proved moderately successful by South American standrards.

The Italian Air Force felt that the Ca.135 was limited by its comparative lack of power. Caproni proposed a version with two 910 hp Hispano Suiza 12Y-31 engines, but the air force refused to consider a version with French engines as France and Italy were at loggerheads. Early in 1938, therefore, the Ca.135 Tipo Spagna airframes were revised for trials with a pair of uprated engines of 1,000 hp, either Fiat A.80 RC.41s or Piaggio P.XI.RC.40 radials. Trials revealed that the Fiat engines were unreliable and failed to improve performance as significantly as the Piaggio engines, so the few Fiat powered bombers were withdrawn from front-line service and transferred to bomber schools.

The Piaggio engine aircraft proved altogether more successful, especially after they had been revised with a new nose section that was somewhat cleaner than the Ca.135 Tipo Spagna. The opportunity was also taken to replace the original Breda dorsal turret with a more modern Caproni-Lanciani turret. Despite its improved performance, the Ca.135/P.XI was not ordered by the Italian air force, which currently preferred tri-motor bombers such as the CANT Z.1007 and SM.79. The Imperial Japanese army air force also evaluated the type in competition to the Fiat BR.20 and preferred the latter.

Thus production of the Ca.135/P.XI was undertaken for a sole export customer, which was the Hungarian air force that received 100 of the type in 1939 and 1940. These aircraft operated with limited success against the Soviets in 1941 and 1942 once Hungary allied itself with the Axis and committed its forces to operations on the Eastern Front.

The last development of the series was the Ca.135bis/Alfa that was fitted with a dihedraled tailplane and was powered by two 1,400 hp Alfa Roemo 135 RC.32 Tornado radial engines. This prototype had a maximum speed of 301 mph, but it was clear by this time that the development potential left in the Ca.135 was marginal and no production was authorized.

Article by JDG

Sources:
Elke Weale, Combat Aircraft of World War II, Bracken Books, 1977.
Wings: Midway to Hiroshima CD-Rom.
Hans Werner Neulen, In the Skies of Europe: Air Forces Allied to the Luftwaffe, 1939-45, Crowood Press, 2000.