WASHINGTON — The U.S. Air Force has accepted the first delivery of Boeing's aerial refueling tanker despite outstanding issues with the aircraft, the service said Thursday.

After a two-year delay, which cost the world's largest aircraft manufacturer more than $3 billion, the first four of an expected 179 KC-46 aerial tankers will be delivered to the Air Force by the end of the month.

And yet, design and software issues remain.

Boeing has agreed to foot the bill for software and hardware upgrades for the camera system used in refueling operations and the Air Force will finance the redesign of the tankers' boom, which is used to deliver fuel to an aircraft.

The Air Force will also withhold 20 percent of its payment to Boeing until progress is made on aircraft deficiencies.