The PZL.37 was designed in the mid-1930s at the PZL factory in Warsaw by Jerzy Dąbrowski. The first PZL.37/I prototype, fitted with a single vertical stabilizer, flew on 13 December 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.
PZL.37 Łoś (suomeksi hirvi) oli puolalainen kaksimoottorinen keskiraskas pommikone. Koneen ensimmäinen prototyyppi lensi ensilentonsa 13. joulukuuta vuonna 1936.
Konetta pidettiin yhtenä toista maailmansotaa edeltäneen ajan moderneimmista pommikoneista. Puolan lisäksi Romania käytti konetyyppiä sodan aikana ja sekä Saksa että Neuvostoliitto saivat Puolan valtauksen yhteydessä haltuunsa muutaman koneen
Before the war, the PZL.37B Łoś was one of the world's most advanced 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 three 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-Rhone 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, 12 by Bulgaria, 30 PZL.37Ds and license by Romania and 10, raw materials and parts for next 25 and license by Turkey and, finally, 12 aircraft for Greece.
Length: 12.92 m
Wingspan: 17.93 m
Height: 5.1 m
Wing area: 53.5 m²
Empty weight: 4,280 kg
Loaded weight: 8,865 kg with basic combat loading
Max. takeoff weight: 9,105 kg
Powerplant: 2 × Bristol Pegasus XX radial engines, 723 kW (970 hp) each
Maximum speed: 412 km/h at 2,800 m with 1,995 kg bomb load
Range: 2,600 km with 1,760 kg bomb load and extra fuel tanks
Combat radius: 1,000 km
Service ceiling: 7,000 m
Rate of climb: 4.7 m/s
Wing loading: 166 kg/m²
Armament: 3 x 7.92 mm observer's machine gun: 1 in the nose, 1 in the rear upper station, 1 in underbelly station. Up to 2,580 kg of bombs (18 x 110 kg + 2 x 300 kg).
Basic load 20 x 110 kg = 2,200 kg.
Bomb load while operating from unprepared fields 880–1320 kg
(8, 10 or 12 x 110 kg)
The Polish Air Force started to receive the Łos A in early 1938, with deliveries of the PZL.37B to operational units, slowed by delays in supply of the Pegasus XX engines and other equipment, in the autumn of 1938. On 1 September 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 four bomber escadres of a Bomber Brigade: the 11th, 12th, 16th and 17th escadres (two escadres with nine 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 Karaś.
About 50 remaining PZL.37s were in the reserve XX group, training units or in repairs. Since only a few months were available to train the crews and complete the equipping of the bombers, the planes were not fully ready when the war broke out. For example, the extra internal fuel tanks were not in service, thus the maximum range of the bomber quoted in specifications was in practice not achievable.
Only the PZL.37s of the Bomber Brigade took part in combat. By 1 September, 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. However, because of this move away from developed airfields to short fields with poor surfaces, during the campaign the planes could take off with only a fraction of their maximum bomb load (typically 800 kg or 8 x 100 kg bombs), which limited their effectiveness. During the Invasion of Poland, from 4 September 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 Częstochowa and Radomsko. They suffered heavy losses due to lack of fighter protection, especially because they usually operated in units of no more than three aircraft at a time. The last combat flights took place on 16 September. During the campaign, the combat units were reinforced with several other aircraft, and about 46 PZL.37s were used in combat. Of the Bomber Brigade aircraft, ten PZL.37s were shot down by fighters, five shot down by enemy anti-aircraft artillery, two bombed on the ground and a further ten 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).
Twenty-six or twenty-seven PZL.37s (17 from the Bomber Brigade and ten 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 air force in the 4th Group, consisting of the 76th and 77th bomber escadres. 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 against the USSR from 22 June 1941.
Among others, they first operated in Bessarabia, then they were bombing Kiev and Odessa. Some were lost, mostly due to anti-aircraft fire. Due to a lack of spare parts, the remaining planes were withdrawn from the front in October 1941 and used for training. In 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.