Without fanfare, Boeing Co. (NYSE: BA) on Sunday flew the first of its four prototype KC-46A tankers from Paine Field in Everett, Wash., to Boeing Field in Seattle. The flight lasted three and a half hours.

The test plane is designated as EMD-1, an acronym for engineering, manufacturing and development. EMD-1 is a specially developed version of Boeing’s 767-2C commercial freighter but it does not yet carry any of the aerial refueling gear or other military systems the final version of the plane will have.

Boeing is on the hook to deliver the first finished KC-46As to the U.S. Air Force by the middle of 2016 and to deliver the first 18 finished planes by the end of 2017. All told, the Air Force has ordered 179 of the KC-46A tankers to replace its fleet of KC-135 tankers, which first entered service in 1957.

The other test planes, EMD-2, -3 and -4, will enter the test flight program in 2015, with EMD-2, the first actual KC-46A, scheduled to fly in April. Boeing officials say they are confident that the other three test planes will be flying in 2015 and that the program remains on schedule to deliver the first 18 planes by the end of 2017.

The company took a six-month delay in the program earlier this year due to a problem with a wiring harness. The KC-46A uses a strengthened fuselage from the 767-2C, a strengthened wing from the 767-300ER (larger to accommodate heavier weight) and the 787’s cockpit display system.

According to a Government Accountability Office (GAO) progress report issued in April, the U.S. Air Force is paying about $287 million per tanker and that does not include a parts and maintenance contract. Adding in the value of the long-term parts and maintenance deal would about double the $51.4 billion value of the contract.

ALSO READ: What Does a Boeing 767-Based KC-46 Cost?

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