Production of the Łoś commenced during the winter of 1936-1937.
During 1938, the first 10 serial aircraft were manufactured, designated as the 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.
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 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.
Production of the Łoś commenced during the winter of 1936-1937. During 1938, the first 10 serial aircraft were manufactured, designated as the 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. 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 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.
PZL.37 Łoś