After years of delays, Boeing’s new KC-46 tanker cleared a major hurdle Thursday when the U.S. Air Force accepted the first delivery of the aerial refueling aircraft.

The tanker, which is built in Everett, will be used to refuel other aircraft mid-air during critical missions. It replaces a tanker, the KC-135, which has been in service for more than 60 years.

Boeing ran into delays with the KC-46 after it discovered problems with the redesigned aerial refueling system, which is much more advanced than an older version and uses camera technology to help guide it toward its target.

"We have identified, and Boeing has agreed to fix at its expense, deficiencies discovered in developmental testing of the remote vision system," Capt. Hope Cronin, an Air Force spokesperson said, Thursday.

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Air Force officials last year expressed frustration with Boeing for taking so long to fix the issues. Boeing hoped to have the first aircraft ready for delivery in 2017.

“I am totally in line with them in terms of their sense of frustration, we have it ourselves,” Leanne Caret, president and CEO of Boeing Defense, Space & Security, said in May.

Now that the paperwork has been signed, McConnell Air Force Base in Kansas will receive four KC-46 tankers, all of which are ready for delivery, Boeing said. A formal delivery ceremony could occur before the end of January, the Air Force said.

Four more aircraft are destined for Altus Air Force Base in Oklahoma.

Boeing signed a contract with the Air Force to build 52 of an expected 179 tankers. 

The KC-46 is much more sophisticated than the tanker it is replacing. In addition to fuel, it can carry cargo, passengers, and patients. It can resist chemical and biological attacks and is packed with technology.

The grey aircraft, which are derived from the 767 airframe, have become a common sight at Boeing Field, where they undergo flight testing.