The PZL.37 Łoś (English: elk) was a Polish twin-engine medium bomber, used in the defense of Poland against the Nazi German Invasion in 1939.
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.
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 Bomber Brigade, 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.