Bomber aircraft in the Polish Air Force
The PZL.37 "Łoś" ('moose') was a Polish twin-engined medium bomber designed and manufactured by national aircraft company Państwowe Zakłady Lotnicze (PZL). It is sometimes incorrectly referred to as "PZL P.37", but the letter "P" was reserved for fighters of Zygmunt Puławski's design (such as the PZL P.11).
Upon its introduction to service, it was popularly considered to be not only the most modern and effective weapon then possessed by Poland, but also to be one of the most advanced bombers then operational in the world. From mid-1938 onwards, interest was expressed by various nations in potential export sales of the PZL.37; in response to this highly favourable reception, PZL, being keen to meet the demands, developed additional variants that were intended for the export market, such as the PZL.37C (which was to be powered by a pair of Gnome-Rhone 14N-0/1 radial engines). An improved enlarged derivative, known as the PZL.49 Miś, was also developed for the Polish Air Force.
The Łoś was extensively used in the defense of Poland during the invasion of Poland by Nazi Germany in September 1939. ;ref name="Murphy/McNiece" ; ;/ref ; On 1 September 1939, the Polish Air Force had roughly 86 PZL.37s in total, but less than a half of those actually saw active combat use due to aircraft being used by training units, being in maintenance, or having been held in reserve. The bombers suffered from a high attrition rate due to lack of fighter protection, and the final Polish combat missions were performed on 19 September. In October 1940, around 26-27 of the PZL.37s that had been evacuated from Poland were seized by the Romanian government and 23 of these aircraft were subsequently used by the Royal Romanian Air Force in the invasion of the Soviet Union.
Development.
Background.
Throughout the majority of the interwar period, the dominant military philosophies and practices within the leadership of Poland was that aircraft were of a secondary importance and a separate air-oriented service within the Polish Armed Forces would be of questionable value. Instead, it was decided to develop the nation's aviation assets to principally function as a means of providing aerial support to the newly formed Polish Land Forces. As such, a relatively minimal budget was allocated for aviation matters while no dedicated doctrine in respect to air power was developed, neither was an effective bomber force pursued in any form throughout much of this period and, when such a force was established, it was met with controversy and heavy criticism from officers who were sceptical that Poland required any bomber aircraft at all.
Despite this unpromising lack of interest, a separate service, known as the "Wojska Lotnicze" (Polish Air Force), had been formed during 1918. During the Polish–Soviet War of 1919, there had been proposals mooted for the formation of a specialised bomber group equipped with modified Breguet 14 B2 biplanes; however, these were not quickly followed up upon due to a lack of understanding and appreciation amongst senior officers. During April 1920, the first Polish bomber squadron was stood up at Poznań–Ławica Airport, Greater Poland, equipped with a variety of captured ex-German aircraft such as Gotha G.IVs, AEG G.IVs, and Friedrichshafen G.IIIs. However, following the end of the conflict with the Soviets, this bomber squadron was amalgamated with a reconnaissance-orientated squadron.
During 1920, the Polish Air Force heavily benefited from an extensive re-armament and expansion programme headed by General Włodzimierz Zagórski, under which the formation of dedicated bomber regiments was envisioned. Accordingly, an order was placed for 32 French-built Farman F.68 BN.4 Goliath twin-engined night bombers, with which it was intended to equip and launch these squadrons. An additional pair of army co-operation squadrons were repurposed as light bomber units instead, equipped with French Potez XV B2 aircraft. Unfortunately, Zagórski's reforms were abandoned following the May Coup led by Józef Piłsudski.
Throughout the 1920s and 1930s, some officials within Poland remained interested in the establishment of a national bomber force, in some cases for prestige value rather than in terms of strict military value. During 1927, the Department of Aeronautics authorised the development of the Lublin R-VIII, a large single-engined reconnaissance bomber. In the following year, the department ordered Polish aviation manufacturer Państwowe Zakłady Lotnicze (PZL) to make preparations towards the development of a heavy multi-engined night bomber; while numerous proposals and design studies were made by both PZL and private factories, but none were approved for further development by the department. Instead, it was decided to procure a number of Dutch-built Fokker F.VII to meet the bomber role, the first of which being delivered during 1929, despite condemnation of the aircraft's poor defensive armament, limited payload capacity, and excessive weight.
Emergence.
Despite not being approved for further development, PZL decided to independently continue refining their designs for a multi-engined night bomber. These studies, led by aircraft engineer Wladyslaw Zalewski, developed an advanced concept for an all-metal low-wing cantilever monoplane bomber known as the PZL.3. The bomber was to be powered by an arrangement of four Bristol Jupiter radial engines positioned in tandem pairs and armed with a pair of side-mounted gun turrets in addition to an internally-stowed bombload of 3,000 kg (6,613 lb). However, during December 1930, all work on this concept was abandoned as an economy measure and the construction of a prototype was abandoned. Reportedly, technical data for the PZL.3 was later received by French manufacturer Potez.
During the 1930s, discussions were held regarding a potential replacement of Poland's Fokker F.VIIs. Numerous concepts were explored by the Department of Aeronautics, the most impressive submission was deemed to have been made by Polish aeronautical engineer Jerzy Dąbrowski, who had produced the initial proposals that would lead to the PZL.37 during spring 1934 and submitted to the department in July 1934. The concept was of a very quick monoplane bomber manned by a crew of four, featuring all-metal stressed-skin construction and powered by a pair of radial engines capable of producing 800–1,200 hp. The airframe was aerodynamically clean and was to be capable of very high performance, far in advance of contemporary bombers; wind tunnel testing using scale models of the aircraft were extremely encouraging.
During October 1934, the instruction to proceed with the proposal, initially designated "P.37", was received by PZL. An express priority was placed upon performance over armament and equipment, relying upon its high speed for its defence; as such, only a single 7.7 mm machine gun was allocated to each turret position instead of the twin-guns and 20 mm dorsal cannon previously considered. On 14 April 1935, the department issued its approval of a full-scale wooden mock-up of the aircraft, authorising the construction of a pair of prototypes along with a structural test frame.
During early 1936, the necessity of revising the design's structure delayed the construction of the partially-assembled first prototype. During May 1936, the prototype commenced ground testing and taxiing trials; however, on 15 June, while being rolled out for its intended first flight, an engine failure was sustained prior to the attempt, resulting in its delay. On 30 June 1936, the first PZL.37/I prototype, which was fitted with a single vertical stabilizer, performed its maiden flight successfully. In August 1936, the initial factory tests were completed, after which the first prototype participated in official airworthiness and service trials. These revealed a handful of shortcomings, including rear fuselage flutter, excessive vibration on the instrumentation panel, overheating of the cylinder heads, and criticism of the cockpit's lack of space, however, test pilots were typically highly impressed with the aircraft.
In response to the feedback from the first prototype's testing, several improvements and other modifications were made, some of which were introduced on the original prototype while more extensive changes were introduced upon the second prototype designated as PZL.37/II. This second prototype featured the adoption of twin-vertical stabilizers, a re-worked cockpit and an altered ventral gun position amongst other changes, functioned as a representative aircraft of the anticipated production bomber and performed its first flight during autumn 1936. Suitably impressed, the department accepted the aircraft for production, placing an initial order for 10 pre-production aircraft, as well as bestowing a name upon the bomber, "Łoś".
Production.
Production of the Łoś commenced during the winter of 1936–1937. During 1938, the first 10 serial aircraft were manufactured, designated as PZL.37A; these were furnished with a single vertical stabilizer. The next 19 interim aircraft were constructed to the PZL.37A bis standard, having been outfitted with the newer twin tail configuration. All of these aircraft were powered by the British-developed Bristol Pegasus XII B radial engine, which was produced in Poland under licence.
The main production variant, the PZL.37B (or: Łoś II), was furnished with the twin tail arrangement along with newer Pegasus XX engines. During autumn 1938, production of the PZL.37B commenced for the Polish Air Force. During the initial period of service, 2 prototypes and 6 serial aircraft were lost in separate crashes; these had been caused by several technical problems, most of which involved the rudder seizing (due to being aero-dynamically overbalanced) and consequent total loss of control. Following the implementation of some structural changes, the PZL.37B became a highly reliable aircraft. By the outbreak of the Second World War, roughly 92 PZL.37 bombers had been produced and delivered to the Polish Air Force, and a further 31 were in different phases of production.
Prior to the Second World War, the PZL.37B Łoś was widely considered to be one of the most advanced bombers then operational in the world. It was able to carry a heavier bombload than considerably larger aircraft, such as the British-built Vickers Wellington, though over shorter range and the dimensions of the individual bombs were limited. Being smaller than most contemporary medium bombers, the Łoś was relatively fast and easy to handle. As a consequence of a favourable landing gear arrangement, having been fitted with double wheels, the bomber could operate from austere airstrips as well as rough fields or meadows. Typically amongst bombers of the late 1930s, its defensive armament consisted of only three machine guns, which subsequently proved too weak against enemy fighters in combat.
Export interest and further development.
Starting with a presentation at a salon in Belgrade in June 1938 and in Paris in November, the PZL.37 met with a huge interest. For export purposes, new variants were developed: the PZL.37C with Gnome-Rhone 14N-0/1 radial engines of 985 cv (971 BHP, 724 kW), maximum speed 445 km/h and the PZL.37D with 14N-20/21 of 1,065 cv (1,050 BHP, 783 kW), maximum speed 460 km/h. In 1939, 20 PZL.37Cs were ordered by Yugoslavia, 12 by Bulgaria, 30 PZL.37Ds and license by Romania and 10, raw materials and parts for next 25 and license by Turkey and, finally, 12 aircraft for Greece.
The Belgian company Constructions Aéronautiques G. Renard was granted permission to perform the license production of between 20 and 50 aircraft for the Second Spanish Republic; however, the venture was abandoned due to the victory of the opposing Nationalists in the Spanish Civil War. In addition to the Belgian venture, it is known that Denmark, Estonia, Finland and Iran were all at various stages of negotiation for their own acquisitions of the type. The Polish military were not allowed to establish an arrangement with Iran due to "lack of production abilities". Nevertheless, the outbreak of the war acted to prevent the production of any of these aircraft. At that time, PZL developed the next variant for the Polish Air Force, the PZL.49 Miś, but this was not completed before the war. Possessing slightly bigger dimensions, the Miś ("Bear") was to be fitted with Bristol Hercules II radial engines of 1,350 bhp (1,370 cv, 1,007 kW), capable of a maximum speed of 520 km/h along with the addition of an upper turret.
Design.
The PZL.37 Łoś was a twin-engined all-metal monoplane medium bomber. The first production variant PZL.37A was relatively conventional in layout, being outfitted with low-set wings, a single vertical stabilizer, and a metal-covered airframe. The late A series (designated Abis) and B series aircraft received a double vertical stabilizer which improved flight stability and increased the field of fire of the dorsal gunner. The aircraft was quite small for its bomb load and range; its carrying capacity was achieved in part by a lift-generating, airfoil-shaped fuselage, which was an innovative feature (previously used e.g. on PZL.26 sports plane). It was shorter and had smaller wings than its German and French counterparts but carried comparable bomb load and