EVERETT — The U.S. Air Force will have to wait a little longer for first flight of a new aerial-refueling tanker, which the Boeing Co. is developing at the Paine Field plant.

Boeing had said that first flight of the plane, derived from the commercial 767, would be in late April. But now the KC-46A Pegasus isn’t expected to fly until later this spring or in the summer, according to people familiar with the program. Neither these sources nor Boeing or Air Force officials would specify the cause or extent of the latest delay.

The plane in question is the first full military version of the tanker. A non-military version of the airplane flew in late December. It has been at Boeing Field in Seattle since, undergoing testing.

The program is under pressure to stay on schedule to deliver the first 18 tankers by August 2017.

The company had originally planned to fly the first military version of the tanker by January, but problems in development, particularly with miles of wiring, pushed that date to April.

“Our team is working hard to get ready for that first flight,” said Chick Ramey, a Boeing spokesman for the KC-46 program. He would not confirm the delay but didn’t deny it.

Boeing doesn’t have room for much more schedule slippage if it’s going to meet the delivery date required in the engineering and development contract with the Pentagon, said Daryl Mayer, an Air Force spokesman.

“These delays have eliminated the margin Boeing built into their original schedule and increase the pressure to perform to the revised schedule,” he said.

The program’s first tanker is in Boeing’s Everett factory, where it is undergoing functional testing prior to a fuel system check and pre-flight tests, Mayer said.

The plane has been successfully powered on, he said.

While remaining work “is well scoped and understood, and Boeing continues to make solid progress, it is taking longer than planned, and we won’t proceed to that first flight until this work is safely completed,” Mayer said.

The delay could push back the Pentagon’s decision on giving the green light for full production. The Air Force plans to order 179 KC-46s, worth an estimated $51 billion, as the first of a three-phase plan to replace an aging tanker fleet.

Boeing is working under a fixed-price development contract for the first 18 planes. The deal caps the federal government’s cost at $4.9 billion. Boeing must cover any cost overruns, which in December were estimated to reach as much as $1.5 billion.

Ramey, the company spokesman, said Boeing is still on track to deliver the first 18 tankers by August 2017, as required by the contract.

Dan Catchpole: 425-339-3454; dcatchpole@heraldnet.com; Twitter: @dcatchpole.