In fact, on July 19, 1963, Joseph Albert Walker flew an X-15 rocket plane to an altitude of 105,900m, or more than 100km, past the Kárman line, i.e. into outer space. Compare that to the Indian flight mentioned in this article, which reached an altitude of 65km. In other words, the US had a reuseable space-capable launch vehicle 50 years ago.
"Mostly" is quite a stretch, since the shuttle is completely dependent not only only the fuel in the external tank, but also on the solid rocket boosters. Indeed, the solid boosters composed the majority of the launch weight and provided the majority of the thrust. For the first couple of minutes, the shuttle was mostly just along for the ride (just as the X-37 is completely along for the ride on the Atlas).
The key analogous role I'm referring to is the ability to deploy, capture, and return payloads to Earth, which is shared by the X-37 and the shuttle. It's not a coincidence that the Air Force developed the X-37 when the shuttle retired.
A very HN correction here but 'space shuttle' refers to the whole stack (orbiter and fel tank and SRBs) and what you seem to be referring to as the 'space shuttle' is actually called the orbiter, a component of the space shuttle. But everyone gets it wrong. Mobile makes it difficult to add links but its explained on the wikipedia article for the space shuttle orbiter, a distinct article from the space shuttle.
Could you elaborate on what you think the current deployment of the X-37 has to do timeline wise with the cancellation of the shuttle program?
After Challenger the Air Force cancelled its plan to use the shuttle for satellites and opted instead for more conventional boosters.
I can't remember whether it was after Challenger or Columbia but didn't NASA also stop returning payloads from orbit from safety reasons after one of those disasters?
I'm sure the Air Force is using the X-37 for something, although they're not saying what, but I don't see how whatever it is has to do with the timeline of the cancellation of the shuttle program.
I know that the Air Force began publicly working on the predecessor (X-40) in 1998 (between the Challenger '86 and Columbia '03 disasters). Presumably they were working secretly even earlier. Also, the X-37's first orbital flight was in 2010 and the shuttle's retirement was in 2011. But that's all I know, and mine is uninformed speculation. Apologies if I expressed too much confidence.
Note that the US Air Force has a re-usable winged unmanned spacecraft that plays a somewhat analogous role to the now-defunct Space Shuttle.
https://en.wikipedia.org/wiki/Boeing_X-37
http://www.bloomberg.com/news/articles/2014-10-14/a-supersec...