Based on the airframe of the 767 jet liner, Boeing's tanker will eventually replace many of the Air Force's aging line of KC-135 tankers, which were manufactured during the Cold War.

The KC-46 has suffered technical setbacks, delays and cost overruns, largely stemming from a problem with the "remote vision" system.

Unlike older tankers, boom operators in the KC-46 do not have a direct line of sight to the plane that is being refueled. Instead, they must rely on an elaborate system of cameras and monitors.

In some cases, boom operators have struggled with image quality and accidentally scraped the outside of the plane being refueled.

In a statement, the Air Force said Boeing has agreed to fix problems with the remote vision system at its own expense -- not the taxpayer's.

"The Air Force has mechanisms in place to ensure Boeing meets its contractual obligations while we continue with initial operational testing and evaluation," Air Force spokeswoman Captain Hope Cronin said.

An Air Force official said it could take three or four years to design and retrofit an improved remote vision system.

The Air Force plans to hold a formal delivery ceremony for four new planes at a base in Kansas as soon as this month.

"This is a major milestone for our next generation tanker and will allow our airmen to begin operational testing and flight training," Cronin said.

Boeing in 2011 beat its European rival Airbus to replace the KC-135 with the newer KC-46.

The Air Force plans to buy 179 KC-46s over the course of the program. The first deliveries had been expected in 2017.

Leanne Caret, president and CEO of Boeing's defense operations, said in a statement that the "KC-46A is a proven, safe, multi-mission aircraft that will transform aerial refueling and mobility operations for decades to come."

Boeing has had to pay about $3.5 billion in pre-tax cost overruns in the program, according to Defense News.

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