Officials won't say why or how long it will remain in orbit, but the last mission lasted a record-breaking two years in space. There has not been much information provided by the USA government or by the authorities in the past.
The U.S. military traditionally holds back the details of the X-37B's classified test missions, while providing information about experiments included aboard the uncrewed mini-space shuttle.
The mystery space plane will deploy a small research satellite dubbed FalconSAT-8, which will spend an extensive period in space conducting a number of experiments, Air Force Secretary and head of the newly established U.S. Space Force, Barbara Barrett, said earlier this month. The aircraft that was launched on Sunday - Operational Test Vehicle 6 - carries several USA military and NASA science experiments.
One question that has loomed over the X-37 is whether it has counterspace capabilities and could be used to test offensive technologies.
The X-37B is 29 feet long (nine meters), with a wing span of 15 feet (4.5 meters). It is the 84th for an Atlas V rocket.
The Air Force said OTV 6 will carry more experiments than any of the previous X-37B missions.
Experiments on board are to include studying the effects of space travel on various materials, and the effect of space radiation on seeds. The experiment will convert solar power into radio frequency microwave energy which could then be transmitted to the ground. However, the livestream cut out afterwards due to the extreme secrecy surrounding the capabilities of the X-37B, said ULA chief executive officer Tory Bruno.
Since the first flight in 2010, the secretive space planes had logged a combined 2,865 days in orbit as of Sunday.