In December 1965, a number of B-52Ds underwent Big Belly modifications to increase bomb capacity for carpet bombings. While the external payload remained at 24� 500 pound (227 kg) or 750 pound (340 kg) bombs, the internal capacity increased from 27 to 84� 500 pound bombs or from 27 to 42� 750 pound bombs. The Big Belly modification now created enough capacity for a total of 60,000 pounds (27215 kg) in 108 bombs. Thus modified, B-52Ds could carry 22,000 pounds (9,980 kg) more than B-52Fs. Replacing B-52Fs, modified B-52Ds entered combat in April 1966 flying from Andersen Air Force Base in Guam. Each bombing mission lasted ten to 12 hours with an aerial refueling by KC-135 Stratotankers. In spring 1967, the aircraft began flying from U Tapao Airfield in Thailand which had the advantage of not requiring in-flight refueling. These missions lasted only 2 to 3 hours. On 15 April 1968, a Replacement Training Unit was established at Castle AFB which converted B-52E through B-52H crews to B-52Ds so they could participate in combat in Southeast Asia.

On 22 November 1972, a B-52D (55-0110) from U-Tapao was hit by a SAM while on a raid over Vinh. The crew was forced to abandon the damaged aircraft over Thailand. This was the first B-52 to be destroyed by hostile fire in Vietnam.

The zenith of B-52 attacks in Vietnam was Operation Linebacker II which consisted of waves of B-52s (mostly D models, but some Gs without jamming equipment and with a smaller bomb load). Over 12 days B-52s flew 729 sorties, dropping 15,237 tons of bombs on Hanoi, Haiphong, and other targets.

Thirty-one B-52s were lost in Vietnam, in the course of flying 126,615 combat sorties. Of these, 18 were shot down or damaged beyond repair by ground fire (including surface-to-air missiles). Two B-52Fs were lost in a midair collision during the first Arc Light mission. Seven B-52Gs were lost during Linebacker II, six to SAMs and one to structural failure.

B-52D tail gunners were credited with shooting down two MiG-21 "Fishbeds"; one on 18 December 1972, by SSgt Samuel O. Turner, and one on 24 December 1972, by A1C Albert E. Moore. Turner was awarded a Silver Star for his actions. The last Arc Light mission took place on 15 August 1973 and all B-52s left Southeast Asia shortly after.

Cold War

During the Cold War, B-52s performed airborne alert duty under code names such as Head Start, Chrome Dome, Hard Head, Round Robin, and Giant Lance. Bombers loitered near points outside the Soviet Union to provide rapid first strike or retaliation capability in case of nuclear war.

On 17 January 1966, a fatal collision occurred between a B-52G and a KC-135 Stratotanker over Palomares, Spain. The four B-28 FI 1.45-megaton-range nuclear bombs on the B-52 were eventually recovered. Two of the four bombs had a minor detonation, as the warheads' conventional explosives were set off, with serious dispersion of both plutonium and uranium. The main fuse safety withstood the violent impact and explosion, preventing a nuclear disaster. After the crash, 1,400 tons of contaminated soil were sent to the United States. The crash and the decontamination were too expensive to risk again and ended the airborne alert program. In 2006 an agreement to investigate and clean the pollution after the accident was made between the U.S. and Spain.

On 21 January 1968, another B-52G with four nuclear bombs aboard crashed on the ice of the North Star Bay while attempting an emergency landing at Thule Air Base in Greenland. The resulting fire caused extensive radioactive contamination the cleanup of which lasted until September of that year.

The Yom Kippur War in October 1973 saw the Soviet Union threaten to intervene on behalf of Egypt and Syria. To stop the Soviets, President Richard M. Nixon called on the military to raise its alert level to DEFCON 3. Secretary of Defense Melvin Laird ordered the B-52s to an immediate war footing. Fully armed and fueled B-52s were circling Greenland. The Soviet Union did not become directly involved in the war.

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