Astronaut Michael Collins discusses in Liftoff how people generally expected in the late 1950s that a descendant of the X-15 would be the first spaceship; that

Astronaut Michael Collins discusses in Liftoff how people generally expected in the late 1950s that a descendant of the X-15 would be the first spaceship; that is, something with wings would one day have enough power to reach orbit. Sputnik, and the consequent desire by the US to get someone in space ASAP, preempted the gradual process of ever-faster aircraft in favor of capsules. Without the space race, the X-15B or X-20 or something else would likely have taken an American into space for the first time. It would have been a few years later than Alan Shepard's first flight in 1961, but would probably have been a more sustainable technology.