A week after the Air Force declared its version of Lockheed Martin Corp.’s F-35 jet ready for limited combat operations, the Pentagon’s top tester warned that the U.S. military’s costliest weapons program is still riddled with deficiencies.

A U.S. Air Force F-35A Lightning II Joint Strike Fighter from the 58th Fighter Squadron, 33rd Fighter Wing, Eglin AFB, Fla., escorts a KC-135R Stratotanker from the 336th Air Refueling Squadron, March ARB, Calif., following an aerial refueling mission, May 16, 2013, off the coast of northwest Florida. The 33rd Fighter Wing is a joint graduate flying and maintenance training wing that trains Air Force, Marine, Navy and international partner operators and maintainers of the F-35 Lightning II. (U.S. Air Force photo by Master Sgt. John R. Nimmo, Sr./RELEASED)

“In fact the program is actually not on a path toward success but instead on a path toward failing to deliver” the aircraft’s full capabilities, “for which the Department is paying almost $400 billion by the scheduled end” of its development in 2018, Michael Gilmore, the Defense Department’s director of operational testing, said in an Aug. 9 memo obtained by Bloomberg News.