To sneak through sophisticated enemy air defenses, the service is developing the futuristic B-21 “Raider” bomber: a stealthy flying wing. To fill out its fleet, it is also counting on the B-52 Stratofortress, an aircraft designed in 1948 with a slide rule. So essential is the B-52 to the Air Force’s long-term plans that 76 of them will fly until at least 2050. By then, the youngest will be nearly 90 years old. Some generals say the plane might live to celebrate its centennial, besides outlasting the hairdo named for it in the 1960s, which was later made a band...