The reusable, unmanned craft is designed to operate in Earth orbit for extended periods; its prior missions in 2010 and 2011 lasted 224 and 469 days. The US government has been resolutely quiet about its precise purpose, and would not confirm a launch time.
That has prompted fevered speculation as to the craft's ultimate purpose. Tuesday's launch had been pushed back from October, delayed by two satellite launches. Patrick Air Force Base in Florida gave notice of a hazard from a launch in a window between 15:45 and 22:15 GMT (10:45 to 17:15 local time).
The X-37B craft, designed by aerospace giant Boeing, shares more than just a passing similarity to the now-retired space shuttle. It is just a quarter the size of the shuttle, but is launched on a rocket - the Atlas V. It is coated in thermal tiles to withstand the heat of re-entry, after which it lands on its own gear autonomously.
The stated mission of the craft, according to the US Air Force, is an "experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform".