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Twenty-six or twenty-seven Polish Air Force PZL.37s (17 from the Bomber Brigade and ten training ones) were withdrawn in 1939 to Romania. During October 1940, these aircraft were seized by the Romanian government and 23 of these aircraft would be used by the Romanian Air Force in the 4th Group, consisting of the 76th and 77th bomber escadres.[24] 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 Because of a lack of spare parts, the remaining planes were withdrawn from the front in October 1941; after this, the type was mainly used for training. During April 1944, the 76th escadrille returned to combat, with nine aircraft, but it was withdrawn from the front on 3 May 1944.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.

The PZL.37 Łoś was a twin-engined all-metal monoplane medium bomber.[17] 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.[3]

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)).[3]

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)[18] 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.