Not any more
B-52s had two types of tail guns over their operational life. The A through G models had a radar aimed <a href=http://www.gunners.net/images/photos/B-52/B52gtail.gif">quad 0.50 caliber machine gun setup with the tail gunner riding in the back. They actually managed to shoot down at least one MiG over Vietnam with them. The B-52D on display at the Air Force Academy had MiG kill markings. The H model had a radar aimed 20mm Vulcan cannon for the tail gun with the gunner riding with the rest of the crew. That was probably removed with they eliminated the tail gunner position in 1991.
The value of the tail gun was pretty limited in the age of air-to-air missiles.