When it entered service the Łoś was one of the most advanced bomber aircraft in the world. The PZL. 37 planes were used by Polish Airforce during the September 1939 War against Germany. 26 of them were later withdrawn to Romania, and after that seized and used in the 4th Group of the Romanian Airforce consisting of the 76th and 77th bomber escadres. Approximately 23 were incorporated in the Romanian Air Force. Inexperience with the PZL.37’s handling and its high wing loading, along with to engine faults led to around a third of the 23 actually accepted into service being lost in accidents.

From 22 June 1941 the 15 remaining aircraft were used during the invasion of the Soviet Union. A lack of spare parts forced their withdrawal in October 1941, after which the survivors were used for training.

The 76th escadrille returned to active duty in April 1944 with nine aircraft, but was withdrawn again on 3 May 1944.