The appearance of United States Army Air Forces heavy bombers caused a problem for the German fighter force. The B-17 Flying Fortress in particular was especially durable, and the armament of the Bf 109 and Fw 190 were not adequate for bomber-destroyer operations. The B-17's eventual deployment in combat box formations provided formidable massed firepower from a hundred or more Browning AN/M2 .50 caliber machine guns. In addition, the Luftwaffe's original solution of Zerstörer twin-engine Messerschmitt Bf 110G bomber destroyers, while effective against unescorted Allied bomber formations, lacked maneuverability and were eviscerated by the USAAF's fighter escorts in late 1943 and early 1944.
The Fw 190, designed as a rugged interceptor capable of withstanding considerable combat damage and delivering a potent 'punch' from its stable gun platform, was considered ideal for anti-bomber operations. Focke-Wulf redesigned parts of the wing structure to accommodate larger armament. The Fw 190 A-6 was the first sub-variant to undergo this change. Its standard armament was increased from four MG 151/20s to two of them with four more in two underwing cannon pods. The aircraft was designated A-6/R1 (Rüstsatz; or field conversion model). The first aircraft were delivered on 20 November 1943. Brief trials saw the twin cannon replaced by the MK 108 30mm autocannon in the outer wing, which then became the A-6/R2. The cannons were blowback-operated, had electric ignition, and were belt fed. The 30mm MK 108 was simple to make and its construction was economical; the majority of its components consisted of just pressed sheet metal stampings. In the A-6/R4, the GM-1 (nitrous oxide) Boost was added for the BMW 801 engine to increase performance at high altitude. For protection, 30 millimetres (1.2 in) of armoured glass was added to the canopy. The A-6/R6 was fitted with twin heavy calibre Werfer-Granate 21 (BR 21) unguided, air-to-air rockets, fired from single underwing tubular launchers (one per wing panel). The increased modifications, in particular heavy firepower, made the Fw 190 a potent bomber-killer. The A-7 evolved in November 1943. Two synchronized 13mm (.51 caliber) MG 131 machine guns replaced the twin cowl-mount synchronized 7.92mm (.318 cal) MG 17 machine guns. The A-7/R variants could carry two 30mm MK 108s as well as BR 21 rockets. This increased its potency as a Pulk-Zerstörer (Bomber Formation Destroyer). The A-8/R2 was the most numerous Sturmbock aircraft, some 900 were built by Fiesler at Kassel with 30mm MK 108s installed in their outer wing panel mounts.
While formidable bomber-killers, the armour and substantial up-gunning with heavier calibre firepower meant the Fw 190 was now cumbersome to maneuver. Vulnerable to Allied fighters, they had to be escorted by Bf 109s. When the Sturmgruppe was able to work as intended, the effects were devastating. With their engines and cockpits heavily armored, the Fw 190 As attacked from astern and gun camera films show that these attacks were often pressed to within 100 yds (90 m).
