Boeing is now assembling the last of four KC-46A test aircraft that the company is producing for the US Air Force’s next-generation aerial refueling tanker programme, the Chicago-based company announces.

Boeing says the completion of the fourth test aircraft puts the company on tack to deliver an initial 18 tankers to the USAF by 2017.

“All four test aircraft are moving through production to support our transition to ground and flight testing later this year,” says the company in a media release.

The first flight of a 767-2C test aircraft – the commercial variant of the KC-46 – will take place by the middle of this year and will be followed by the first flight of a KC-46A equipped with refueling systems by early 2015, Boeing says.

The company adds that it expects to deliver the first production aircraft to the USAF in 2016.

The USAF has orders and options to acquire a total of 179 KC-46As through 2027.

The aircraft is derived from Boeing’s 767 passenger jetliner and is powered by two Pratt & Whitney PW4062 turbofans.

In addition to serving as an aerial refueling tanker, KC-46As can be configured to accommodate cargo or up to 114 passengers, or to serve as an aero-medical evacuation aircraft, says Boeing.