The bomber took prominence in the world’s air forces during the 1930s to the point where an air force’s strength was determined almost solely by how many bombers they could field above anything else. This spurred on bomber development often at the negligence of development of other combat aircraft and this was especially true in the RAF where at several points in the 1930s the fastest aircraft in the inventory actually belonged to Bomber Command. As the 1930s drew on, the limitations of the single and twin engine bombers then in service were becoming obvious while the expectations of what a bomber could do to win a war was growing without a solid basis in reality. Towards the end of the 1930s several aircraft designers the world over began work on developing new more powerful four engine bombers that offered to revolutionise air warfare. The four engine bomber had the potential to carry a heavier load over much greater distances and sufficient defensive armament to give the crew the ability to fight its way to bomb the enemy’s military and economic.
The RAF had used four-engine bombers as far back as 1918 with the extraordinary Handley-Page V/1500 but these aircraft had proven extremely complex to operate. Now the technology had matured making them more practical and one of the first four-engine “heavies” to emerge in the late 1930s was the Shorts S.29 Stirling. The Stirling heralded the start of a new generation of heavy bombers for the RAF that included the famed Avro Lancaster and the Handley-Page Halifax as well as several American designs.
Sadly, the Stirling is remembered as being the least successful of the three. Even before the first metal was cut in building the prototype the Stirling was already doomed thanks to pre-war thinking. The Stirling’s altitude performance was curtailed by a limit imposed on the aircraft’s wingspan by the Air Ministry, a decision that was dictated by the need for it to be housed in existing hangars. Another example of this pre-war thinking that would prove problematic for the entire RAF’s combat force was the need for the defensive armament to comprise of .303 (7.7mm) machine guns, the same calibre as the Army’s rifles, in order to ease the burden on the logistical chain. While the logic behind both these decisions is obvious, both caused problems for the Stirling when it came to defending itself against enemy fighter attack (while RAF fighters adopted 20mm cannons, Bomber Command’s aircraft carried on firing .303 rounds until the end of the war).
Despite these shortcomings the Stirling still offered a dramatic increase in performance over existing twin engine “medium” bombers such as the Bristol Blenheim and the Handley-Page Hampden. While it may not have done as much as the Lancaster or Halifax to bring Nazi Germany to its knees the Stirling laid the ground work for its compatriots to follow up with. The Stirling first flew in May 1939, entering service a year later and after a brief combat career as a bomber the aircraft found its niche as a glider towing aircraft. A great many number of the troop- and supply-gliders launched on D-Day were towed by Stirlings and it is this mission which was perhaps its greatest contribution to the war effort.
In Europe the UK seemed to be almost the only nation that had a genuine interest in four engine strategic heavy bombers by the late 1930s with a solid plan in place to develop and build a force of them. Germany, for the time being at least, remained committed to their force of twin-engine “Blitz bombers” such as the Dornier Do.17, Heinkel He.111 and Junkers Ju.88. These were excellent designs of their breed being generally superior to similar British designs but were still limited to primarily supporting the Army having the range and warload to attack tactical targets ahead of the frontlines but were generally unsuited a strategic aerial warfare. The Germans expected war to come in around 1945 by which time they would have their own new four engine strategic bomber designs but Britain and France declaring war in 1939 in response to the invasion of Poland meant that those dreams would never be truly realised.
The Axis powers (Germany, Italy and Japan) did have a handful of four engine aircraft in service but they were never as prominent as the twin engine designs. The most well-known was the Focke-Wulf Fw200 Condor maritime (Right) patrol aircraft which Winston Churchill famously once referred to as the “Scourge of the Atlantic”. However, it would actually be Mussolini’s Italy that would take an early lead in developing a four engine strategic bomber for use against the Allies.
The Piaggio P.108 is little known outside of its home country but there were high hopes placed upon it when it was conceived by Italian aircraft designer Giovanni Casiraghi in the late 1930s. Casiraghi had been inspired by developments in the United States having spent several years working there and seeing the plans for future strategic bombers then in development most notably the Boeing B-17 Flying Fortress. Upon returning home he began work on his own four engine bomber design and designed the Piaggio P.50; a large all-wood monoplane design with four engines mounted on two mounting brackets in a push/pull arrangement. First flying in 1937, the Italian government proved sceptical and no order was placed.
Casiraghi returned to the drawing board and adapted the P.50 in to an all-metal design with the four engines spaced out along the wing in an all-tractor arrangement. Initially designated the P.50-II the changes became such that the finalised design was granted its own designation and the P.108 was born in 1938. The Italians weren’t satisfied with having just a bomber however and Casiraghi developed a number of variants for different uses. This resulted in four distinct variants;
- 108A – Anti-ship variant
- 108B – Bomber
- 108C – Airliner
- 108D – Military transport
In the end only the B and C models were ever built beyond the prototype stage and even then only in small numbers. The aircraft’s combat record was even less impressive than the Stirling’s despite the advantages it offered over other Italian bombers such as the Savoia-Marchetti SM.79 and most of the P.108B bombers ended their days as transports but was it better or worse than the British aircraft?
As with all bomber comparisons this article will look at which aircraft could best carry out the strategic mission. It must be noted that the Stirling was never envisioned as operating in a day bomber role but the P.108B eventually adopted a night role because of the density of Allied fighter opposition during daylight hours.
The Stirling was powered by four Bristol Hercules II 14-cylinder radial engines each of which produced a respectable 1,375hp. Because of the size of the pistons they were arranged in two rows rather than one row as in smaller engines. It was one of the first radial engines to feature a single-sleeve valve configuration which optimised intake and exhaust gas flow which allowed higher compression ratios. The engine was air-cooled which was advantageous for bomber operations since liquid cooled engines were more susceptible to being damaged by shrapnel but weren’t as powerful as later Merlin engines as fitted to the Lancaster which went some way to explaining the Stirling’s poor altitude performance.
The P.108 was one of the few aircraft types of World War II to be powered by engines designed and built by the same company responsible for the airframe. The aircraft was powered by four Piaggio P.XII air-cooled 18-cylinder radial engines that generated an impressive 1,500hp at take-off. The engine achieved this figure by essentially being two French Gnome-Rhône 9K Mistral engines that Paiggio built under license as the P.X (confusingly the Gnome-Rhône 9K Mistral was itself a licensed version of the British Bristol Jupiter).
This coupling of two engines to produce a single more powerful unit reflects the state of Italian aero engine technology in the late 1930s/early 40s which was lacking behind British and German technology. This would ultimately lead to Italy acquiring German engines such as the Daimler-Benz DB 600 series for their fighters. In this instance the forcing together of the two engines in to one unit (which means the P.108 could technically be considered an eight-engine aircraft) resulted in shocking unreliability that resulted in the loss of several aircraft on operations when they were being used to their fullest. In January 1943, serviceability amongst the remaining aircraft dropped to just 2-3 machines forcing their withdrawal from operations.
The Stirling had a maximum range of 2,330 miles depending on the size of the bombload that was carried. Typically, the aircraft flew missions from the UK as far afield as Berlin in East Germany and Northern Italy necessitating a smaller warload (q.v.). The aircraft had a top speed of 282mph and its comparatively high power-to-weight ratio and thick wing made it one of the best handling of all the RAF’s four engine types. A major source of contrition as far as military planning was concerned regarding the Stirling was its low service ceiling especially when carrying a heavy bombload. The aircraft had a maximum (empty bomb bay) altitude of just 16,500ft – 17,000ft thanks in no small part to the relatively thick wings and short wingspan dictated by the pre-war decision to make the aircraft fit in existing hangars. This low service ceiling meant that the aircraft had to fly around the Alps instead of over them unlike the Lancasters and Halifaxes.
The potential the Piaggio P.108B offered the Axis powers was obvious despite the hindrance of the unreliable P.XII engines. The aircraft’s maximum range was only marginally inferior to the Stirling but compared to other common Axis types such as the Savoia-Marchetti SM.79 (1,600 miles) and the German Heinkel He.111 (1,429 miles) it was superior with range being in the region of 2,187 miles. The P.108B had quite an impressive service ceiling, especially when compared to the Stirling, of 27,000ft but where the P.108B suffered was in its top speed. Able to reach 260mph at 14,500ft (depending on bombload) it was considerably slower than the B-17 Flying Fortress, Stirling, Avro Lancaster and Handley-Page Halifax which increased the time over enemy territory and the chances of being intercepted.
A bomber’s capabilities in the Second World War was judged primarily by how many bombs could be carried and how far. The Stirling had a 14,000lb maximum theoretical bombload although this was rarely achieved since it imposed such hefty penalties on the aircraft’s performance particularly with regards to range that a more modest load of around 4,000lbs was the norm. This was similar to earlier twin engine types such as the Vickers Wellington but the Stirling could carry them further and faster. The bomb bay was also unsuitable for carrying some of the bigger weapons the RAF began fielding later in the war.
A typical bombload for the P.108B was higher being in the 7,000lb figure although generally the weapons used were smaller than the Stirling’s 500lb weapons. This did have a knock-on effect with range and when on the longest ranged missions (2,000 miles) a bombload of around 4,000lbs was carried.
The Stirling featured three defensive weapon stations in a typical British arrangement for a night bomber. A powered nose turret featured two Browning .303 (7.7mm) machine guns while a tail turret featured four; this demonstrated the importance of rear hemisphere protection when facing night fighters. The night fighters the aircraft was expected to face such as the Messerschmitt Bf110 and Junkers Ju88 used primitive radar and infra-red equipment to peer through the night after being guided to the aircraft’s vicinity by ground control radar stations. Given the narrow viewpoint this equipment offered they were almost always guided in to a tail-chase position where it would be easier to detect the bomber because of the lower overtake speeds giving the radar operator more time to set up an attack. The Stirling also featured a dorsal turret with another pair of two .303 machine guns which were usually operated in conjunction with the tail turret to attack night fighters in the upper-rear hemisphere.
The Piaggio P.108B featured six separate defensive weapon stations. A powered nose turret featured a 12.7mm Breda machine gun while a second 12.7mm gun was mounted in ventral retractable turret for defending against attack from below. There were two waist gunner positions covering both flanks each equipped with a single 7.7mm machine gun. The aircraft lacked a dedicated tail turret but Casiraghi circumvented this with two unique radio controlled turrets equipped with two 12.7mm machine gun positioned on the outer engine nacelles. Theoretically, these provided excellent protection against fighters attacking from the rear hemisphere trying to hit the engines. In practice however they suffered chronic reliability problems, were difficult to aim and affected airflow.
Both these aircraft have their strengths over the other but overall the Piaggio P.108B was generally superior or at least equal to the Stirling in terms of performance with the notable exception of it being marginally slower. It has to be remembered however that the Stirling did generate higher sortie rates than the Piaggio design as a result of the P.XII engines which proved to be the Italian bomber’s Achilles-heel.
Comparing the two aircraft’s ability to defend themselves is somewhat problematic since the P.108B was expected to operate in daylight hours as well as the night while the Stirling was intended to operate exclusively as a night bomber. The P.108B therefore was designed with a greater field of fire with which to fend off enemy fighters although it’s hitting power remained relatively light especially in the rear and forward quadrants when compared to the Stirling which although it had lighter calibre weapons, had a greater concentration of them. The Stirling’s dorsal turret meant it had better protection against attack from above but the P.108B’s ventral turret gave it better coverage from below. The P.108B’s interesting nacelle turrets could have caught more than one unsuspecting Allied pilot out had they worked as promised.
(Images sourced from Wings Pallet & Commons.Wikimedia)