First deliveries of the B-534 to the Czechoslovak air force began in October 1935, and 568 had been completed by 1939. The first 100 of these were of the first series. The second prototype was the blueprint for the I series, although it was built with an open cockpit. These early series aircraft were initially armed with four 7.92mm vz.28 guns. Two were located in the nose on either side of the engine in a similar manner to the Avia B-34 and two were fitted in the wings. At an early stage of production, it was however recognised that the wing-mounted guns were troublesome. Aircraft from serial number B-534.47 were completed without the wing guns, which were also removed from the earlier aircraft. The first to fourth series aircraft were fitted with the Avia licence-built version of the Hispano-Suiza 12Ydrs engine. This was a liquid-cooled V12-cylinder engine with a capacity of 36.05 litres. On the ground its normal power rating was 650 hp, it could deliver 750 hp for two minutes. At an operational height of 3,000 m (9,842 ft), it could deliver 850 hp for 30 minutes. At an operational height of 4,000 m (13,123 ft), it could reach 860 hp. Total fuel was 347 litres (76 gal) which was held in two fuselage fuel tanks of 90 and 257 litres.