The PZL.37 Łoś was a twin-engined all-metal monoplane medium bomber.
It was relatively conventional in layout, being outfitted with low-set wings, a twin tail (on the majority of aircraft), and a metal-covered exterior.
The aircraft was quite small for its bomb load and range; its capacity was achieved in part by a lift-generating, airfoil-shaped fuselage, which was another then-unusual feature (previously used e.g. on PZL.26 sports plane).
It was much shorter and had smaller wings than many German and French counterparts; it was slightly larger than the Lockheed Model 10 Electra Amelia Earhart used.
The crew consisted of four: pilot, commander-bombardier, radio operator and a rear gunner.
The bombardier was accommodated in the glazed nose, with a forward machine gun.
The radio operator sat inside the fuselage, above the bomb bay, and he also operated an underbelly rear machine gun.

The Łoś featured retractable main undercarriage, which retracted into purpose-built alcoves located within the engine nacelles.
The undercarriage was double-wheeled, complete with independent suspension for each wheel.
The bomber was powered by a pair of Bristol Pegasus radial engines; the PZL.37A model had Pegasus XII B engines (normal power: 860 BHP (873 cv, 642 kW), maximum: 940 BHP (953 cv, 701 kW)), the PZL.37B variant had Pegasus XX engines (normal power: 905 HP (918 cv, 675 kW), maximum: 970 BHP (984 cv, 723 kW)).

Originally, it was intended for a popular wing profile, designed by Ryszard Bartel (also used in many other successful Polish airframes incl.
PZL P.11) to be used for the aircraft; however, the requirement of providing bomb storage with the internal space of the wings necessitated modification to the design.
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) 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.
The modified profile was viewed as highly successful and was later re-used for other projects (sometimes in further modified form; e.g. PZL.46 Sum, PZL.49 Miś, PZL.50).

The bomber's offensive payload was spread across a two-section bomb bay set within the fuselage (providing space for up to 4 bombs) and a total of 8 compact bomb bays located in the central section of the wings (which had space for a maximum of 16 bombs).
This arrangement of bomb bays imposed considerable restrictions on the types of bombs that could be carried, especially in the wing bays which were quite small due to the need to fit between the ribs of the wing.
The maximum load was 2,580 kg (2 × 300 kg and 18 × 110 kg).
Apart from a pair of 300 kg bombs in one of the fuselage bomb bays, it could not carry bombs larger than 110 kg.
When flown at the maximum loadout, the majority of the weight of the bombs was carried inside the wings.
There were no provisions for mounting bombs on the outside of the aircraft.
During the invasion of Poland in 1939, 110 kg was the maximum weight used, since the 300 kg bombs were available only in small quantities, and were difficult to load at improvised airfields with little infrastructure.
 50 kg bombs were also used.
Maximal bomb load taken from soft surface runways was reduced to about 800–1200 kg.
The PZL.37 Łoś was a twin-engined all-metal monoplane medium bomber. It was relatively conventional in layout, being outfitted with low-set wings, a twin tail (on the majority of aircraft), and a metal-covered exterior. The aircraft was quite small for its bomb load and range; its capacity was achieved in part by a lift-generating, airfoil-shaped fuselage, which was another then-unusual feature (previously used e.g. on PZL.26 sports plane). It was much shorter and had smaller wings than many German and French counterparts; it was slightly larger than the Lockheed Model 10 Electra Amelia Earhart used. The crew consisted of four: pilot, commander-bombardier, radio operator and a rear gunner. The bombardier was accommodated in the glazed nose, with a forward machine gun. The radio operator sat inside the fuselage, above the bomb bay, and he also operated an underbelly rear machine gun.
The Łoś featured retractable main undercarriage, which retracted into purpose-built alcoves located within the engine nacelles. The undercarriage was double-wheeled, complete with independent suspension for each wheel. The bomber was powered by a pair of Bristol Pegasus radial engines; the PZL.37A model had Pegasus XII B engines (normal power: 860 BHP (873 cv, 642 kW), maximum: 940 BHP (953 cv, 701 kW)), the PZL.37B variant had Pegasus XX engines (normal power: 905 HP (918 cv, 675 kW), maximum: 970 BHP (984 cv, 723 kW)).
Originally, it was intended for a popular wing profile, designed by Ryszard Bartel (also used in many other successful Polish airframes incl. PZL P.11) to be used for the aircraft; however, the requirement of providing bomb storage with the internal space of the wings necessitated modification to the design. 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) 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. The modified profile was viewed as highly successful and was later re-used for other projects (sometimes in further modified form; e.g. PZL.46 Sum, PZL.49 Miś, PZL.50).
The bomber's offensive payload was spread across a two-section bomb bay set within the fuselage (providing space for up to 4 bombs) and a total of 8 compact bomb bays located in the central section of the wings (which had space for a maximum of 16 bombs). This arrangement of bomb bays imposed considerable restrictions on the types of bombs that could be carried, especially in the wing bays which were quite small due to the need to fit between the ribs of the wing. The maximum load was 2,580 kg (2 × 300 kg and 18 × 110 kg). Apart from a pair of 300 kg bombs in one of the fuselage bomb bays, it could not carry bombs larger than 110 kg. When flown at the maximum loadout, the majority of the weight of the bombs was carried inside the wings. There were no provisions for mounting bombs on the outside of the aircraft. During the invasion of Poland in 1939, 110 kg was the maximum weight used, since the 300 kg bombs were available only in small quantities, and were difficult to load at improvised airfields with little infrastructure.  50 kg bombs were also used. Maximal bomb load taken from soft surface runways was reduced to about 800–1200 kg.
PZL.37 Łoś