The Avia B-71 was a license-built Czechoslovak variant of the Tupolev SB. This variant saw service with Bulgaria.
In September 1939 Bulgaria bought 32 B-71 from Germany. Those machines were originally built at the GAZ 22 factory, in Fili, Moscow oblast, and were delivered to the Ceskoslovenské letecvo (CL - Czechoslovak Air Force) in March 1939, just before the German Occupation.
The Germans handed the B-71s to the Vazdushni Voyski (VV - Royal Bulgarian Air Force) at Merseburg, Germany and were not operational until early 1940, when they were assigned to the 5. Bombardirovochen Polk (5.BP - 5th Bomber Regiment), based at Plovdiv. The B-71 received the nickname of "Zherav" (crane) from Bulgarian pilots.
The Bulgarian B-71s saw combat for the first time in late September and early October 1941 when Greeks in the Bulgarian-occupied region of Thrace revolted. Three B-71s took off from Plovdiv and bombed the Greek town of Drama, in north-western Macedonia. This turned to be the only combat mission of the 5.BP during the period that Bulgaria was allied with Germany.
On 5th September 1944 the USSR declared war on Bulgaria (which, until then, had remained neutral in the Axis war against the USSR) and the Soviet Army invaded three days later. During this period, the pro-Soviet Bulgarian faction called "Democratic Party of the Patriotic Front" led by Kimon Georgiev, seized power and declared war on Germany on 9th September.
In November 1944, the 2. Yato (Squadron) of the 5.BP had a total of 21 B-71 on strength. On 14th November the 5.BP was redeployed from Plovdiv to Vrazhdebna, close to Sofia, from where they took part in various bombing missions against the retreating Wehrmacht. On these missions, the B-71s had not a precise targets, but where instructed to take upon targets of opportunity in certain pre-defined areas where the VV thought German troops were concentrated.
The Bulgarian B-71s were heavily employed over Yugoslavia. On 18th November three B-71s attacked a German train at the Vuchitran train station, causing heavy damage to the train. The next day four B-71s bombed the Zhitarica railroad station. Although they were meet with heavy anti-air fire, they returned safely to Vrazhdebna.
Two days later, on 20th November, several B-71s flew a 'free hunting' mission against targets of opportunity in the regions of Mitrovica and Rashka, attacking a train north of Mitrovica at 12:30 hours. In this bomb run Zherav 4's bomb bay doors malfunctioned and couldn't release its bombs and Zherav 11, flew by Sgt. Manol Dojchev became disoriented and had to force-land at Staro Zhelezare, close to Plovdiv.
The fourth and final bombing mission of the Bulgarian B-71s took place the next day, on 21st November when four B-71s joined a formation of approximately 15 Lockheed P-38 of the US 15th Air Force. The B-71s bombed a motorized German column near Kamenica while the P-38s attacked a railroad station near Mitrovica. This constituted the only joint Bulgarian-American action against Germany during World War 2. During this action Zherav 4 lost its bearings and had to force-land at Shtarklevei.
After this attack the front moved beyond the range of the B-71s. During their service period with Bulgaria, they made a total of 14 sorties during those four missions in which they destroyed a total of two railroad stations, three trains and one German motorized column, at the cost of no Bulgarian airman killed or wounded.
Sources:
1st Signal Squadron - Aircraft In action 194 - Tupolev SB in Action
2nd https://en.wikipedia.org/wiki/Tupolev_SB
3rd https://massimotessitori.altervista.org/sovietwarplanes/pages/sb/tapani/b-71/bulgarian/bulgarian.htm