Though originating as a Czech pre-war design (through the Avia B.35 venture), the B.135 was utilized during World War 2 (1939-1945) solely under the banner of the Bulgarian Air Force. The type was a modern, low-wing monoplane originally developed for the Czech Air Force to a 1935 requirement by the Ministry of National Defence. Three progressive B.35 prototypes emerged - two prior to the German occupation of March 1939 and the third after. The Bulgarian Air Force flew the B.35/2 prototype during November of 1939 and followed this in June of 1940 with trials of the B.35/3. The latter was accepted as the B.135 to which the German Air Ministry allowed the aircraft to be locally produced for Bulgaria out of Lovech. The Bulgarian Air Force commissioned for twelve of the fighters along with a stock of engines (62).
The B.135 retained many of the qualities ironed out in the final B.35/3 prototype. The original prototype's construction involved wood wings but this was given up in the finalized form to carry all-metal wings. Standard armament became 1 x 20mm cannon firing through the propeller hub and 2 x 7.92mm machine guns in the wings. The undercarriage was retractable while of the "tail-dragger" arrangement. The pilot sat under an enclosed canopy with generally adequate views of the surrounding action - the position of the wing mainplanes (forward of midships) and the raised fuselage spine did little to help.
Beyond the original twelve on order, a further fifty examples were to emerge from Bulgarian lines under the designation of DAR ("Darzhavna Aeroplanna Rabotilnitsa") 11 "Ljastuvka" ("Swallow") to help shore up Bulgarian air strength. The first twelve units arrived during the middle part of 1942 though the engine portion of the procurement foundered and left just 35 delivered. This forced Bulgarian reliance on the German Messerschmitt Bf 109 fighter crop.
Most of the B.135's wartime use was in training primarily due to unreliable engines. At least four participated in an operational-level manner during a March 30th, 1944 interception sortie which yielded a downed enemy bomber - the only known air kill of the war by the B.135 series.
Total production of the B.135 stood at the original twelve units.
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