Boeing's tanker will eventually replace the Air Force's aging line of KC-135 tankers, which were manufactured during the Cold War.

But the KC-46 has suffered setbacks 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 a system of cameras and monitors.

In some cases, boom operators have struggled with depth of field and image quality, and accidentally scraped the skin 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.

Even though the plane is not fully operational, pilots and airmen can still train with it, she added.

"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.

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

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