On 13th December 1936 there was made a test flight of a bomber PZL-37 Łoś. In spite of initial difficulties the aircraft turned out to be very successful and modern construction. PZL-37 Łoś was a two-engine low-wing with all-metal construction. It could carry a load over 2.5 t of bombs, the weaponry of this aircraft consisted of 3 machine guns. Until the outbreak of war a total production run of PZL-37 Łoś was 120. They were used in Poland and Romania. In Polish campaign in 1939 the bombers were used by the Bomber Brigade to raid the Wehrmacht divisions.