According to the Air Force fact sheets, the B-52A first flew in 1954. B-52B models began operational service in 1955. The last one built, the B-52H was delivered in 1962. The H models are the only ones still flying. According to Air Force numbers, there are still 85 in active service and 9 in the reserves. Long service life, for sure. When my father was in the Air Force, they had them (1959-1963) and they were in use in my day as well (1984-1988). I don't think Boeing ever envisioned them being in service for anywhere near that long.