The PZL P.37 Los (Elk) was the last in the trio of Polish combat aircraft which were among the most modern in the world during the 1930s. It was designed by a team led by Jerzy Dabrowski but despite being probably the finest bomber in the world at the time, was subject to severe criticism by the Army chiefs who sought to disrupt production and training of this superb aircraft. As a result, service entry of the P.37 was delayed and only a trickle had escaped production lines at the time of the German invasion of which over half were eventually destroyed in action during the first two weeks. The remaining aircraft were eventually evacuated to Romania where they went on to serve against the USSR with some being retained as target tugs until the 1950s. Previously, Bulgaria, Yugoslavia, and Turkey had made pre-war orders of the P.37 which were not completed.

First flight of the P.37/I prototype took place in June 1936 and was based on two previously aborted projects, the P.3 bomber and P.30 civil transport. Production versions for the Polish air force began with the P.37a with a single fin and rudder which was later changed to twin units in the P.37abis. These were followed by the P.37b with more powerful engines and a revised cockpit. Lastly, the P.49 was designed with up to 1600-hp engines but was still in the prototype stage when the Germans invaded. Planned export variants included the P.37c and P.37d both of which featured Gnome-Rhône enignes.