The PZL.37 was designed in the mid-1930s at the PZL factory in Warsaw by Jerzy Da;browski. The first prototype, fitted with a single vertical stabilizer, flew on December 13, 1936. The second prototype PZL.37/II, with twin vertical stabilizers and other improvements, was accepted for production. The first 10 serial aircraft were produced in 1938 as the PZL.37A variant with a single vertical stabilizer, however. The next 19 interim aircraft were built as PZL.37A bis, with a twin tail. They all were powered by Bristol Pegasus XII B radial engines produced in Poland under licence.

The main production variant, the PZL.37B (or: Łoś II), was fitted with the twin tail and newer Pegasus XX engines. Production of PZL.37B for the Polish Air Force started in autumn 1938. During the initial period of PZL.37 service, 2 prototypes and 6 serial planes, were lost in crashes caused by technical problems, mostly with rudders. After some structural changes, the PZL.37B became a fully reliable aircraft. By the outbreak of World War II, about 92 PZL.37s had been produced and given to the Air Force, and a further 31 were in different phases of production.

Before the war, the PZL.37B Łoś was one of the world's most modern bombers. It was able to carry a heavier bombload than similar aircraft, for example the Vickers Wellington though the size of the bombs was limited. Smaller than most contemporary medium bombers, it was relatively fast and easy to handle. Thanks to a landing gear with double wheels it could operate from rough fields or meadows. Typically for the late 1930s, its defensive armament consisted of only 3 machine guns, which proved too weak against enemy fighters.

Starting with a presentation at a salon in Belgrade in June 1938 and in Paris in November, the PZL.37 met with a huge interest. For export purposes, new variants were developed: the PZL.37C with Gnome-Rhô:ne 14N-0/1 engines of 985 cv (971 BHP, 724 kW), maximum speed 445 km/h and the PZL.37D with 14N-20/21 of 1,065 cv (1,050 BHP, 783 kW), maximum speed 460 km/h. In 1939, 20 PZL.37Cs were ordered by Yugoslavia, 15 by Bulgaria, 30 PZL.37D by Romania and 25 by Turkey and Belgium ordered a license. Also several other coutries were negotiating. The outbreak of the war prevented the production of these aircraft. At that time, PZL developed the next variant for the Polish airforce, the PZL.49 Mis', but this was not completed before the war. Having slightly bigger dimensions, Mis' ("Bear") was to be fitted with Bristol Hercules II engines of 1,350 BHP (1,370 cv, 1,007 kW), maximum speed 520 km/h and an upper turret.

Technical design - Source: Wikipedia
The aircraft was conventional in layout, all metal, metal-covered, with low-set laminar-flow wings and a twin tail. In size it was slightly larger than the Amelia Earhart's Lockheed L-10 Electra. 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 main undercarriage retracted into the engine nacelles. The undercarriage was double-wheeled, with an independent suspension for each wheel. The plane was powered by two Bristol Pegasus radial engines. The PZL.37A had Pegasus XII B engines (normal power: 860 BHP (873 cv, 642 kW), maximum: 940 BHP (953 cv, 701 kW)), the PZL.37B had Pegasus XX engines (normal power: 905 HP (918 cv, 675 kW), maximum: 970 BHP (984 cv, 723 kW)). The bombs were carried in two-section bomb bay in the fuselage and 8 bomb bays in th e central section of the wings. The maximum load was 2,580 kg (2 × 300 kg and 18 × 110 kg). Apart from two 300 kg bombs, it could not carry larger bombs, than 110 kg (during the Invasion of Poland 1939, 110 kg was the maximum size used)

Operational history - Source: Wikipedia
The Polish Air Force started to receive the PZL.37B in the spring of 1939. On September 1, 1939, it had about 86 PZL.37s in total, but less than a half of those were used in combat. 36 PZL.37Bs were in 4 bomber escadres of a Bomber Brigade: the 11th, 12th, 16th and 17th escadres (two escadres with 9 aircraft each, constituted a group, in Polish: dywizjon; the PZL.37 were in groups X and XV). The rest of the Bomber Brigade aircraft were PZL.23 Karas'. About 50 remaining PZL.37s were in the reserve XX group, training units or in repairs.

Only the PZL.37s of the Bomber Brigade took part in combat. By September 1, they had been deployed to rural improvised airfields, so they were not destroyed on the ground by the Germans in their initial attack on the main Polish airbases. During the Invasion of Poland, from September 4 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 16th Armoured Corps near Cze;stochowa and Radomsko. They suffered heavy losses due to lack of fighter protection, especially, that they usually acted in no more, than three aircraft at a time. Last combat flights took place on September 16. During the campaign, the combat units were reinforced with several other aircraft, and about 46 PZL.37s were used in combat. Of Bomber Brigade, 10 PZL.37s were sh ot down by fighters, 5 shot down by enemy anti-aircraft artillery, two bombed on the ground and a further 10 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).

26 or 27 PZL.37s (17 from the Bomber Brigade and 10 training ones) were withdrawn in 1939 to Romania. In October 1940 they were seized by the Romanian government and 23 were next used by the Romanian airforce in the 4th Group, consisting of the 76th and 77th bomber escadres. Some were uparmed with 4 machineguns (the Polish PWU machineguns were still used). About 1/3 were lost in crashes due to lack of experience of Romanian pilots with PZL.37 handling and its high wing loading, and due to engine faults. About 15 were used against the USSR from June 22, 1941. Among others, they first operated in Bessarabia, then they were bombing Kiev and Odessa. Some were lost, mostly due to anti-aircraft fire. Because of lack of spares, the remaining ones were withdrawn from the front in October 1941 and used for training. In April 1944, the 76th escadre returned to combat, with 9 aircraft, but it was withdrawn from the front on May 3, 1944. After Romania joined the allies, on September 1, 1944 the G erman aircraft destroyed 5 PZL.37s on a ground.

Captured planes were also tested in Germany and the USSR. Not many PZL.37s, however, fell into German hands (probably only two), because Polish workers scrapped about 30 PZL.37s remaining in factories in Okęcie and Mielec in October 1939, under pretext of cleaning up the area, before the competent German authorities were able to reconnoitre. There are no surviving PZL.37 aircraft currently.