The B-52A was the first to be fitted with defensive armament - the A-3A Defensive Fire Control System (DFCS) was operated by a tail gunner, seated in the rear of the plane underneath a transparent canopy. The system utilized a battery of four .50 caliber M3 machine guns in a hydraulically controlled and gyroscopically stabilized gun turret in the extreme tail. Each gun had 600 rounds of ammunition. The system also employed separate search and track radars and provided automatic lead angle computation for the guns. As an alternative, the gunner also had an optical gun sight for manual operation of the guns.
Nine of the first ten RB-52Bs (52-004/008 and 52-010/013) used the A-3A DFCS. RB-52B (52-0009) was fitted with the alternative MD-5 DFCS which incorporated a pair of M24A-1 20-mm cannons in place of the quad .50 calibers. This system was adopted as standard equipment on the remaining 17 RB-52Bs and 16 B-52Bs (52-8710/8716 and 53-0366/0391). The new system proved to offer no real improvement and the last seven B-52Bs reverted back to the original A-3A system of four machine guns with a supposedly improved A-3A DFCS. However, in reality, many of the problems remained.
The defensive armament of the B-52C comprised a battery of four .50 caliber machine guns, just as it had on most of the B-52Bs. All but one of the B-52Cs used the supposedly improved A-3A DFCS that had been used on the last seven B-52Bs. Unfortunately, the A-3A was itself less than fully reliable, and the last B-52C (54-2688) was fitted with an improved MD-9 DFCS which continued to utilize the quad .50 caliber gun turret.
The B-52D through F models all incorporated the MD-9 DFCS.
In the B-52G, the gunner was moved from the rear of the aircraft to a position beside the electronic warfare officer in the forward part of the fuselage. He was provided with a rearward-facing upward-firing ejector seat. A new Avco-Crosley AN/ASG-15 DFCS was fitted in the tail to support the now remotely-operated, rearward-firing gun turret. Like earlier versions of the DFCS, the AN/ASG-15 featured separate radar dishes for search and track, but it also carried a television camera. The gunner could operate the .50 caliber guns by using the AN/AGS-15 DFCS in an auto mode that used the radars to search and track targets and a computational system to aim the weapons, or manually where he monitored approaching threats either by radar or by closed-circuit television and aimed the weapons with hand controls.
The B-52H uses the AN/ASG-21 DFCS utilizing the GE M61A1 20mm cannon; a 20-mm six-barrel rotary cannon. Though both the AN/ASG-15 and AN/ASG-21 were designed to accomplish the same task, they were nonetheless very different in their design and operation. The ASG-15 had more than 20 modes of operation while the ASG-21 had just four. The AN/ASG-15's .50 caliber machine guns were also prone to jam while the AN/ASG-21 GE weapon rarely did. Similar to the DFCS employed on the B-58 Hustler, the B-52H's AN/ASG-21 featured two radars in its DFCS. In the mid-1980's, Emerson upgraded the ASG-21 to a totally digital system with the exception of three tubes.