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. 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.
Despite this unpromising lack of interest, a separate service, known as the Wojska Lotnicze (Polish Air Force), had been formed during 1918.
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.
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. 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.
During the 1930s, discussions were held regarding a potential replacement of Poland's Fokker F.VIIs.
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. 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
thumb|The second prototype PZL.37/II
Production of the Łoś commenced during the winter of 1936–1937.
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.
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. 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 defensive armament. The crew consisted of four: pilot, commander-bombardier, radio operator and a dorsal gunner. The bombardier was accommodated in the glazed nose, with a forward-firing machine gun. The radio operator sat inside the fuselage, above the bomb bay, and also operated the ventral machine gun during combat. which the aircraft simply lacked. However, the profile did display lower drag than expected and the initial PZL.37A version possessed a maximal speed 10 per cent higher than the originally planned 360 km/h. it was followed by deliveries of the improved PZL.37B to operational units, which had been slowed by delays in supply of the Pegasus XX engines and other equipment, during the autumn of 1938.
On 1 September 1939, the Polish Air Force had roughly 86 PZL.37s in total, but less than a half of those were used in combat. 36 PZL.37Bs were in four bomber escadres of a Bomber Brigade: the 11th, 12th, 16th and 17th escadres (two escadres with nine aircraft each, constituted a group, in Polish: dywizjon; the PZL.37 were in groups X and XV). The remainder of the Bomber Brigade's aircraft were PZL.23 Karaś. About 50 remaining PZL.37s were assigned to the reserve XX group, training units or in repairs. As a consequence of there only having been a few months available to train the crews and complete the equipping of the bombers, the planes were not fully ready when the war broke out. For example, the extra internal fuel tanks for the type were not yet in service, thus the maximum range of the bomber that had been quoted in specifications was in practice not achievable. However, because of this move away from developed airfields to short fields with poor surfaces, during the campaign the planes could take off with only a fraction of their maximum bomb load (typically 800 kg or 8 x 100 kg bombs), which limited their effectiveness. During the invasion of Poland, from 4 September onward the planes of the Bomber Brigade were attacking German armoured columns in day attacks, forced by the desperate situation to perform this mission for which they were not designed (the original plans to bomb targets inside Germany were quickly abandoned). Most notably, they hampered the advance of the XVI Army Corps near Częstochowa and Radomsko. During the campaign, the combat units were reinforced with several other aircraft, and about 46 PZL.37s were used in combat. Of the Bomber Brigade aircraft, ten PZL.37s were shot down by fighters, five shot down by enemy anti-aircraft artillery, two bombed on the ground and a further ten lost in other ways. A number of not fully completed, training or reserve PZL.37s were also destroyed on airfields and in factories (18 PZL.37s were bombed in a reserve base in Małaszewicze and in a factory in Warsaw – Okęcie). Some were uparmed with four machine guns (the Polish PWU machineguns were still used). About one third were lost in crashes due to lack of experience of Romanian pilots with the PZL.37's handling and its high wing loading, and due to engine faults. About 15 were used in combat against the Soviet Union from 22 June 1941. Among others, they first operated in Bessarabia, while they were later used to conduct bombing missions targeting Kiev and Odessa. Some of the bombers were lost on these missions, mostly due to anti-aircraft fire. Because of a lack of spare parts, the remaining planes were withdrawn from the front in October 1941; after this, the type was mainly used for training. During April 1944, the 76th squadron returned to combat, with nine aircraft, but it was withdrawn from the front on 3 May 1944. After Romania joined the Allies, on 1 September 1944, German aircraft destroyed five PZL.37s on the ground during retaliatory attacks against Romanian targets.
Additionally, a number of captured planes underwent testing by both Nazi Germany and the Soviet Union. Relatively few PZL.37s fell into German hands, probably only two aircraft; this is partially due to the efforts of Polish factory workers who scrapped roughly 30 PZL.37s that remained in factories in Okęcie and Mielec, under the pretext of cleaning up the area, during October 1939, before the German authorities were able to reconnoiter.
Preserved parts and replicas
thumb|1:1 scale model of PZL.37 Łoś in Mielec
thumb|upright|Bristol Pegasus engine used by PZL.37 Łoś on display at the [[Polish Aviation Museum in Kraków.]]
There are no surviving PZL.37 aircraft.
An original PZL Pegaz 20 engine of the type used in PZL.37 Łoś is in the collection of the Polish Aviation Museum in Kraków. This engine was sent from Poland to the United States in the spring of 1939 to be exhibited at the 1939 New York World's Fair and thus survived the war, and was returned to Poland in 2006.
A 1:1 scale non-flying model of PZL.37 Łoś was assembled at PZL Mielec factory, in the factory hall used to construct these bombers before the war. The dimensions of the model were determined only from photographs as very little of the original blueprints for the aircraft survive; nevertheless, the model's external shape matches the original very closely. It was constructed from aluminium and steel leftovers from aircraft under construction at the factory and was presented to the public in September 2012. The interior of the plane was not reproduced, except the bombardier's position in the nose. Currently the model is exhibited outdoors on the grounds of the Mielec factory.()
A number of crash sites of PZL.37 Łoś shot down in September, 1939 have been identified and aircraft parts have been recovered from them. One crash site has a simple 1:1 model of the aircraft displayed as a monument.
Variants
;PZL.37/I
:First prototype with a single tail fin.
;PZL.37/II
:First prototype with a double tail fins and improvements.
;PZL.37A
:First 10 serial aircraft with single tail, Bristol Pegasus XIIB engines.
;PZL.37Abis
:Batch of 19 serial aircraft of A version with double tail fins.
;PZL.37B version I and II
:Main production variant with double tail fins and PZL Pegaz XX engines.
;PZL.37C
:Planned version with Gnome-Rhône 14N-01 engines.
;PZL.37D
:Planned version with Gnome-Rhône 14N-21 engines.
;Planned:
;
- Belgian Air Force – Belgium bought a licence to build an unknown number in 1938, none could be built before the fall of Belgium in 1940.
;
- Bulgarian Air Force ordered 12 PZL.37Cs. The start of World War II did not allow them to be delivered.
;
- Royal Hellenic Air Force ordered 12 aircraft.
; Spanish Republic
- Spanish Republican Air Force; not delivered.
;
- Turkish Air Force ordered ten PZL.37Ds, materials for the next 25 and license rights.
;
- Royal Yugoslav Air Force ordered 20 PZL.37Cs.