The U.S. Air Force’s new KC-46A aerial refueling tanker made by Boeing Co. is scheduled to make its first flight on Sept. 25, a general said.

The date was announced Tuesday by Brig. Gen. Duke Richardson during the Air & Space Conference near Washington, D.C. The milestone for the eventual successor to the KC-135 and KC-10 was initially planned for the spring.

“Once that first flight occurs we’ll go into initial air worthiness,” he said. That means the second flight will begin testing the boom, hose and drogue systems, he said.

Richardson, the program executive officer for tankers with the Air Force Material Command at Wright Patterson Air Force Base, Ohio, said the subsequent flights will involve a variety of aircraft flying with the KC-46 and culminate with actual refueling flights in January.

Sen. John McCain, a Republican from Arizona and chairman of the Senate Armed Services Committee, said the recent cost overrun on the aircraft, known as the Pegasus and based on the 767 twin-engine commercial airliner, is “deeply unfortunate” and that he’s concerned about delays to the program.

McCain said he detailed the concerns in a recent letter to Defense Secretary Ashton Carter. He and Sen. Jack Reed, a Democrat from Rhode Island, sent a similar letter over issues with the Air Force’s new bomber program.

“While the recently announced cost overrun on the Air Force’s KC-46A tanker is deeply unfortunate, it is encouraging that the contractor, and not the taxpayer, will bear this expense,” McCain said.

“That said, the resulting delays to the program’s internal deadlines for completing key qualification and planned ground and flight testing activities are indicative of a program at risk of not meeting its planned delivery milestones,” he said.

Boeing plans to deliver the first 18 KC-46As to the Air Force by August 2017. The service estimates it will spend $49 billion to develop and build 149 of the planes to replace its aging fleet of KC-135s, according to Pentagon budget documents. Boeing forecasts an $80 billion global market for the new tankers, according to Trading Alpha.