 The modified B-52Fs were the only SAC bombers to deploy for Arc Light missions until 1966, when the B-52Fs were replaced by B-52Ds with the Big Belly modification than enabled them to carry a larger and more varied bomb load.Later B-52 crews were sent through an intensive two-week course on the B-52D, making them eligible for duty in Southeast Asia.B-52Ds assigned to combat duty in Vietnam were painted in a modified camouflage scheme on the upper surfaces with the undersides, lower fuselage, and both sides of the vertical fin being painted in a glossy black.