The Bulgarian air force was largely inactive until mid-1943, when the U.S. Army Air Forces (USAAF) conducted their first major bombing raid on the Axis-held oil fields of Ploesti, Romania. The Bulgarians' attempt to engage the U.S. B-24 bombers clearly showed that their air arm was hopelessly obsolete. Most of the fighters could not catch the bombers, and Bulgarian aircraft shot down only 2 B-24s. The Germans then provided the Bulgarians with 120 French Dewoitine D.520 fighters and additional Messerschmitt aircraft.
These additional planes did little to improve the effectiveness of the Bulgarian air force. Between December 1943 and January 1944, Allied bombers sortied virtually unmolested over Bulgaria, and their defensive cover of P-38 fighters shot down 39 Bulgarian fighters with negligible losses of their own. Introduction of the superior P-51 Mustang fighter further reduced the effectiveness of Bulgarian air defenses.
After it was invaded and occupied by the Soviet Union in September 1944, Bulgaria switched sides in the war. Royal Bulgarian Air Force units then provided ground support for the Red Army in Yugoslavia and Hungary.
Eric W. Osborne
Tarnstrom, Ronald L. Handbooks of Armed Forces: Balkans, Part II. Lindsborg, KS: Trogen Publications, 1984.