Putting a prototype 707 through a barrel roll seems like an absolutely crazy idea. It's a huge airplane that was certainly not designed for acrobatics, but that didn't stop Boeing test pilot Tex Johnson from doing it anyway during a demonstration flight over Seattle in 1955:
As Tex points out, contrary to what it looks like, this is a completely safe, hazard-free maneuver, because it's "1g." In other words, the entire maneuver takes place under one gravity, which is the same amount of force that you're feeling right now. While it's true that the airplane does end up completely sideways (twice) and upside-down (once), because of the way that the roll is performed, the force on the airplane remains constant as the acceleration vector changes. So, when the airplane is sideways, it's also accelerating sideways. When the airplane is upside-down, it's accelerating downwards. Throughout the entire roll, the airplane is accelerating in such a way that there's always one gravity of force pulling at it in the direction that it was designed for. As far as the aircraft is concerned, it's really not much different than flying straight and level: the stresses on the airframe aren't significantly increased, air is still moving over the control surfaces, and fuel is still flowing into the engines.
The 707 was not the only big airplane that pulled this off: the Concorde was barrel rolled multiple times during its testing. Heck, someone even did it with a prototype Avro Vulcan bomber at the Farnborough Air Show back in the 50s:
And if you're very very good, you can even pour yourself a drink while completely upside-down:
Via Futility Closet