The U.S. Air Force's KC-135 Block 45 upgrade program reached a milestone after the branch modified its 45th tanker.

Under the program, the tanker aircraft received new liquid crystal displays, autopilot, computer module updates, radio altimeters, and digital flight directors. Air Force officials say the program aims to keep the legacy aircraft in service.

"This is the capstone modification that takes your 1950s/1960s-era tanker and makes it a 21st century asset that's as modern as any flight deck we have in the Air Force," Legacy Tanker Division commander Col. Mark Mocio said in a press release. "It enables us to meet all the global rules we have on air traffic navigation now and for a long time to come."

The $910 million upgrade program began nearly seven years ago to extend the service life for 135 of the tankers. Boeing delivered the last batch of the aircraft to the Air Force in 1961.

KC-135 Stratotankers are powered by four turbofans, and are capable of carrying up to 83,000 pounds of cargo. The tankers transfer fuel to partner aircraft through a flying boom.

In addition to refueling missions, the aircraft can also be used to transport litter and support medical evacuations.