The Delta IV medium rocket will carry a national security payload designed, built and operated by the NRO, the agency said.
Due to the mission's classified nature, little is known about the NROL-47 payload, but it is believed to be radar reconnaissance satellite, operating from a retrograde orbit inclined 123 degrees.
Launch has been canceled.
Friday's launch was originally supposed to happen on Wednesday, but was pushed to Thursday due to high winds. NROL-47 will join the Future Imagery Architecture (FIA) satellite constellation, which flies under the code name Topaz.
If you see something like this in the sky this afternoon, don't panic. It performs launch services for the US government at Vandenberg and Cape Canaveral Air Force Station in Cape Canaveral, Florida.
If the launch is successful, it will be the 27th launch for the NRO and the 36th launch of the Delta IV rocket since it was first launched in 2002, according to ULA. The common booster core was powered by an RS-68A liquid hydrogen/liquid oxygen engine.
The launch is carrying a classified spy satellite called NROL-47. The forecast shows a 90 percent chance of favorable weather conditions for launch.
With more than a century of combined heritage, United Launch Alliance is the nation's most experienced and reliable launch service provider. But this time, it has used the two-stage, 217-foot-tall (66 m) Delta IV rocket.
Colonel Greg Wood, 30th Space Wing vice commander, said: "We are ready and eager to take on this Delta launch".
A ground system helium problem caused blastoff to be delayed by more than hour. The commercially developed EELV program supports the full range of government mission requirements, while delivering on schedule and providing significant cost savings over the legacy launch systems.