Boeing B-52 Stratofortress B-52D

The B-52D was the fourth production version of the Stratofortress and the first the US Air Force ordered in genuinely large numbers. The A, B and C series had served to mature the bomber and equip Strategic Air Command's first squadrons; with the D, the USAF settled on a pure long-range bomber configuration and bought 170 aircraft, the second-largest total of any variant. The first B-52D flew in May 1956 and deliveries began that year, the first aircraft being accepted in December 1956.

That volume forced Boeing to open a second production line. As B-47 Stratojet manufacture wound down, the Wichita plant in Kansas was phased into the programme: Seattle built 101 B-52Ds and Wichita the remaining 69, and both factories went on sharing the E and F series. It was the first time the Stratofortress came off two assembly lines at once.

As built, the B-52D was virtually identical to the preceding B-52C. It kept the same Pratt & Whitney J57-P-19W or J57-P-29WA turbojets, water-injected and rated at 10,500 lbf (47 kN) each, and the same MD-9 fire-control system for the tail turret. The main difference was one of concept: the D dropped the option of carrying the reconnaissance capsule that the RB-52B and C could still fit in the bomb bay, and was intended purely as a bomber. For escape, the pilots and the electronic countermeasures operator ejected upwards while the lower-deck crew ejected downwards; the tail gunner, still housed in the conventional position at the end of the fuselage, had to jettison the gun assembly before bailing out, a limitation that only disappeared with the B-52G.

Over time the D was adapted to the low-level penetration profile SAC adopted in the late 1950s, when Soviet surface-to-air missiles made high-altitude bombing untenable. It gained a tail-clearance radar, Doppler navigation and low-altitude radar altimeters, together with structural reinforcement that allowed it to operate "down in the weeds". The Big Four programme, launched in November 1959 and completed in 1963 across the operational fleet apart from the early Bs, also added the ability to launch the AGM-28 Hound Dog stand-off missile and the ADM-20 Quail decoy, plus a more advanced countermeasures suite.

On 26 September 1958 a B-52D set a world speed record of 560.705 mph (902.367 km/h) over a 10,000-kilometre closed circuit without payload; the same day another B-52D set a second record of 597.675 mph (961.865 km/h) over a 5,000-kilometre circuit. The early years also brought losses. On 11 February 1958, 56-0610 crashed short of the runway at Ellsworth AFB, South Dakota, after a total loss of power on final approach: two of the eight crew and three ground personnel were killed, and investigators found that frozen fuel lines had clogged the filters. On 23 June 1959, 56-0591, nicknamed "Tommy's Tigator", came down in the Ochoco National Forest near Burns, Oregon, during a Boeing test flight after turbulence caused the horizontal stabiliser to fail; all five company personnel died. On 15 December 1960, 55-0098 collided with a KC-135 during refuelling; the boom pierced the wing and, on landing at Larson AFB, the starboard wing failed and the aircraft caught fire during the landing roll, although the whole crew got out. On 13 January 1964, 55-0060, callsign "Buzz 14", lost its vertical stabiliser in a winter storm and crashed on Savage Mountain, Maryland, with two Mk 53 bombs aboard, which were recovered relatively intact; four of the five crew ejected, but two died of exposure.

The decisive transformation of the B-52D came in late 1965 with the Big Belly modification, designed for carpet bombing over Vietnam. The external load remained 24 bombs of 500 or 750 pounds, but the internal load rose from 27 to 84 500-pound bombs, or from 27 to 42 750-pounders, for a total of 108 bombs and some 60,000 pounds (27,000 kg): 22,000 pounds (10,000 kg) more than a B-52F could carry. The Pima Air & Space Museum likens that load to nine Second World War B-17s. Intended to replace the Fs, the modified Ds entered combat in April 1966 from Andersen Air Force Base on Guam, flying missions of ten to twelve hours with one KC-135 refuelling; from spring 1967 they also operated from U-Tapao in Thailand, where no tanker support was needed. Aircraft sent to Southeast Asia wore camouflage with black undersides to defeat searchlights, and the Rivet Rambler modification of 1971 gave them Phase V countermeasures that were better than those of many later versions. For all these reasons the D was the most extensively used model of the war.

On 22 November 1972, 55-110 from U-Tapao was hit by a SAM over Vinh and the crew had to abandon it over Thailand: it was the first B-52 destroyed by hostile fire. The climax came with Operation Linebacker II, from 18 to 29 December 1972, flown mostly by B-52Ds alongside some Gs that lacked jamming equipment. In twelve days the bombers flew 729 sorties and dropped 15,237 tons of bombs on Hanoi, Haiphong and other targets; fifteen B-52s were shot down, five were heavily damaged and five more suffered medium damage, and 25 crewmen were killed. On the first night alone, the 68 missiles fired by North Vietnamese batteries brought down two Gs from Andersen and one D from U-Tapao, while two badly damaged Ds limped into Thailand. Over the whole war 31 Stratofortresses were lost, ten of them shot down over North Vietnam.

B-52D tail gunners scored the only air-to-air victories ever credited to the type. On 18 December 1972 Staff Sergeant Samuel O. Turner opened fire with his quad .50-calibre guns on a MiG-21 closing in after a bomb run; the fighter exploded behind the bomber, as confirmed by Master Sergeant Louis E. Le Blanc in another aircraft, and Turner received the Silver Star. On 24 December, aboard "Diamond Lil" bound for the rail yards at Thái Nguyên, Airman First Class Albert E. Moore fired on another MiG-21 from 4,000 yards until it vanished from his scope, while Technical Sergeant Clarence W. Chute watched it burn from a neighbouring bomber. Hanoi never acknowledged those kills, though Vietnamese sources credit the B-52 with a third victory on 16 April 1972. The last Arc Light mission was flown on 15 August 1973.

While the E and F series were withdrawn between 1967 and 1973, the USAF chose to extend the life of the D. In the mid-1970s, 80 aircraft went through the Pacer Plank programme, completed in 1977, which replaced the lower wing and fuselage skins and renewed structural components. The fleet remained largely intact until late 1978, when 37 aircraft that had not been upgraded were retired; the rest left service between 1982 and 1983, the last operational examples being withdrawn in October 1983.

Several B-52Ds are preserved. The Pima Air & Space Museum in Arizona displays 55-0067 "Lone Star Lady" in the markings of the 7th Bombardment Wing at Carswell AFB, Texas, in 1981. Turner's 56-676 is shown at Fairchild AFB in Spokane with its kill marking, and Moore's "Diamond Lil" stands at the US Air Force Academy in Colorado.