The most successful Allied bomber of the Great War had a rather ungainly appearance:
It is overshadowed by Russian pioneering multi-engine heavies, by German Gothas and giant Staaken bombers, by British Handley Page designs, as well as by a good dozen of other impressive aircraft that never flew or were tested after the Armistice. In fact, the Italian Caproni bombers had made several long-range bombing raids on Austro-Hungarian targets well before the Handley Page O/100 had even flown, taking heavier payload than the famous (and much larger) Sikorsky bomber.
In 1913 pioneer aircraft designer Gianni Caproni (seen on the picture above with Sub.-Lt. Laureato) designed a bomber of advanced concept powered by three engines, all contained in a central crew nacelle. Two engines drove tractor propellers by means of extension gearing and the third a pusher propeller at the rear of the nacelle. There was little official interest in the design and no construction was undertaken.
However a Caproni prototype bomber flew for the first time in October 1914 with a 74.5kW Gnome rotary engine at the rear of the central nacelle driving a pusher propeller and two 59.6kW Gnomes conventionally mounted on the lower wings driving tractor propellers. Two booms extended aft of the wing engines to support the triple rudder tail-plane. The Italian government was slow to grasp the possibilities of the new design, but eventually a contract was signed for 12 Caproni 300HP bombers powered by three 74.5kW Fiat A.10 engines. A further 150 were delivered during the next two years. These aircraft were also known as Ca.1s and were followed by nine Ca.350HP machines (with the third engine replaced by a 112kW Isotta Fraschini) designated Ca.2.
Ca.3 (or Ca.33) was in fact the alternative designation of the Ca.450HP, 299 of which were built between 1917 and 1919. Powered by three 112kW Isotta Fraschini V.4B engines, the Ca.3 proved the most successful Allied bomber of World War I, taking part in many outstanding raids.
Normal bomb load was 200kg and defensive 7.7mm machine-guns were mounted over the nose cockpit and in a precarious open position which comprised a tubular metal structure incorporating a ladder leading from the rear of the central nacelle (just in front of the engine) to a gun ring just behind and slightly higher than the trailing edge of the upper wing centre section:
Ca.3s equipped three bomber groups by the end of 1917 and made attacks in numbers on Pola and Kotor naval bases, railway junctions and troop concentrations. License production was undertaken in France by Robert Esnault-Pelterie, 83 examples (designated CEP) being completed.
They made damaging night raids on targets along the Marne as well as attacking troop concentrations near Amiens. In Italy Ca.3s hindered the Austrian advance from the Piave and gave strong support to the final victorious Italian offensive (see Poetic Bombers).
With the revival of Italian air power by Mussolini and the formation of the Regia Aeronautica in 1923, the Ca.3 design was given a new lease of life. A handful of surviving Ca.3s bore witness to the longevity and effectiveness of the design and so orders were given to place it back in production in the Ca.3mod or Ca.36 versions. A number of minor improvements and production simplifications were incorporated and 153 were built over three years. Eighteen Ca.3 mods took part in the 1925 military manoeuvres. The same year eight machines were sent to Libya in support of the reconquest of that territory. By the end of 1926, however, the last examples had been withdrawn from first-line service.
Two other Caproni designs were also emloyed by the Allies during WWI. One is the Ca.5, Caproni's attempt to significantly uprate the power on the existing Ca.3 design, with some versions of the Ca.5 eventually carrying engines with nearly five times the total power that the first Ca.1 had.
Apart from greater power, various refinements were made to the design, including modifications to the main nacelle and undercarriage, and completely new wings. The first prototype flew in late 1917 and the type remained in production until 1921. Some 659 of all versions were built by Caproni, and another three were licence-built in the US (two Ca.44s by Standard, and one Ca.46 by Fisher). Plans for licence production in France did not eventuate.
The Ca.5 was a three-engine biplane of a wooden construction, covered with fabric. The crew of four was placed in an open streamlined central nacelle (front gunner, two pilots and rear gunner-mechanic). The rear gunner manned upper machine guns, standing upon the central engine in a protective "cage", just before a propeller.
Armament consisted of two to four Revelli 6.5 mm or 7.7 mm machine guns, one in front ring mounting and one, two or sometimes even three in an upper ring mounting. Bombs were suspended under the hull.
Another design was much larger aircraft, a triplane that shared the Ca.3/5 unusual layout. The Ca.4 was a three-engine, twin-boom triplane of a wooden construction with a fabric-covered frame. The open center nacelle was attached to the undersurface of the center wing. It contained the pusher engine, pilot, and forward gunner. At least one variation of the center nacelle seated the crew in a two-seat tandem format with the forward position a gunner/pilot and the rear position the pilot. Others used a forward gunner with side by side pilot positions to the rear of the gunner. Two rear gunners were positioned one in each boom behind the center wing. An engineer or second pilot could also be accommodated there.
Ca.4s were tested by the Italian Air Force in 1917 and began operations in 1918. They were used for attacking targets in Austro-Hungary. In April 1918, 6 Ca.42s were used by the British RNAS (No. 227 Sqn). At least three CA.42s were sent to the USA for evaluation. After the war, the Ca. 4 was replaced in Italy by the Ca.36.
Despite its unstable and fragile appearance, the Ca.4 was well-designed. Its size, without regards to its height, was not any larger than that of other foreign heavy bombers. With Liberty engines, it had a fast speed, similar to other heavy bombers, while its bomb load had one of the largest capacities of that era, surpassed only by that of the Imperial German Zeppelin-Staaken R.VI. If it had been flown with other engines, its performance would have suffered.
Note: there is some variation in published sources over early Caproni designations. The confusion stems, in part, from three separate schemes used to designate these aircraft - Caproni's in-house designations of the time, those used by the Italian Army, and designations created after the war by Caproni to refer to past designs.