The P.11 served as Poland's primary fighter defence throughout the 1930s, including during the Polish campaign of 1939 by neighbouring Nazi Germany. However, as a consequence of the rapid advances in aircraft design during the late 1930s (seen in such fighters as the Messerschmitt Bf 109), it was outclassed by its rivals at the onset of the war. The majority of the Polish Air Force's P.11s were destroyed during 1939; however, it is believed that as many as 36 were flown to Romania and were subsequently taken over by the Romanian Air Force.