The PZL.37 Łoś ('moose') was a Polish twin-engined medium bomber designed and manufactured by national aircraft company Państwowe Zakłady Lotnicze (PZL).

[3] 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.

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.

[4] 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.

[4] 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.

[4] 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.

[5] 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.

[6] An additional pair of army co-operation squadrons were repurposed as light bomber units instead, equipped with French Potez XV B2 aircraft.

[7] 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.

[8] 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.

[10] 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.

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.

[12] 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.

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.

[12] 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.

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.

[3] 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.

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.

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 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).

Some publications claim the resulting profile displayed laminar-flow properties (one of the first in the world), but this is disputed – its shape which resembled laminar flow wings developed in later years, but this was largely fortuitous, and at any rate achieving the laminar flow regime would have required additional construction features (most importantly extremely smooth wings shaped with extreme precision)[a] 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.

[2] During early 1938, the Polish Air Force started to receive the Łos A variant;[20] 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.

[3] During these engagements, the PZL.37 suffered heavy losses as a result of a lack of adequate fighter protection, which was further amplified by the operational tactics employed, usually flying missions in units of no more than three aircraft at a time.