Fifteen years ago they were talking about extending BUFF to 2010.....I'd fairly recently read about some of them being expected to serve past 2020.....but 2040?! Holy bat droppings, Batman! Those airplanes are antiques already; the examples flying now were built in the 1960's (G's and H's, I think, with most of the G models gone now).
On the other side of the chessboard, we see a lot of Russian designs being extended in service, too, including several Antonov transports and the Tu-95 "Bear". Not nearly as true for fighters and other high-G airframes, but you'd think all that vibration would put "Bears" and "Coots" and "Curls" out of business by now. IIRC, the Ilyushin Il-76 "Classic" jet transport is a 30-year-old design now, too, as are some examples of Boeing airliners, especially the 747 workhorse. Some 727's and 737's in third-world, second-tier airline service also have 60's build dates and high hours.
Aircraft, like cars, seem to be lasting a lot longer than they used to, 50 years ago. But a proposed retirement date after 2040 for the B-52 still sounds absolutely fantastic.
That was a unconfirmed rumor date. I have heard with advancements with new cruise missiles and new anti-radar technology that they would extend until 2040.