Pentagon officials have been preparing a misleading assessment of progress on Lockheed Martin Corp.'s F-35, the costliest U.S. weapons program, the Defense Department's chief tester warned.

"If not changed, the existing responses would at best be considered misleading and at worst, prevarications," Michael Gilmore, the DOD's director of operational test and evaluation, wrote in an internal memo criticizing the draft response to questions about F-35 testing from Senate Armed Services Committee Chairman John McCain.

Gilmore's memo is the latest example of his vocal doubts about the F-35's performance in key tests. His critiques are at odds with the Pentagon's narrative that the program is on course after earlier problems. President-elect Donald Trump and his defense secretary -- he's nominating retired general James Mattis -- will have to decide next year whether to increase F-35 production to 70 in fiscal 2018 from 63 this year, as requested by the Defense Department.

Trump, who on Tuesday complained that the cost of the new Air Force One being built by Boeing "is totally out of control," has also raised some questions in the past about the F-35. In an October 2015 interview with radio host Hugh Hewitt, Trump criticized the fighter's cost and said he heard "that it's not very good" and that "existing planes are better."

In a Nov. 3 letter to departing Defense Secretary Ash Carter, McCain, an Arizona Republican, said he was "extremely disappointed to learn of another delay" in the $57 billion development and demonstration phase of the F-35 "with an associated cost overrun that may be upwards of $1 billion."

Several of the answers in the draft response to McCain "ignore acknowledged facts, are ambiguous and misleading and if signed and sent as-is" could "generate substantial issues with the Congress," Gilmore wrote to Frank Kendall, undersecretary of defense for acquisition, in the Nov. 28 memo obtained by Bloomberg News.

The draft answers should "be revised to provide clear, accurate and complete answers," said Gilmore, who also raised concerns about the F-35 in a Nov. 18 letter to Deputy Defense Secretary Robert Work.

Gilmore "has shared his concerns" with Kendall who "has them for advisement in his response to Senator McCain," spokesman Mark Wright said in an email. Navy Lt. Cmdr. Courtney Hillson, a spokeswoman for Work, said in an email that Gilmore's information "was and continues to be used to help senior leaders make informed decisions."

Gilmore challenged passages in the Defense Department's draft response to McCain that assert:

- The F-35's development phase is due to end in "early 2018." Gilmore said the department should "state clearly that development flight testing will not complete -- at the earliest" -- until mid-2018.

- Operational combat testing that all weapons systems must pass will start in mid-2018 and be completed a year later. Gilmore labeled that "false." Instead, he said the tests will commence "no sooner than late 2018, or, more likely, in early 2019 but could be as late as 2020."

- An Air Force certification to lawmakers that F-35s delivered in fiscal 2018 will have full combat capability remains "valid." Gilmore said that is "highly unlikely" because of delays in testing the critical final version of the plane's software and correcting 276 pending deficiencies.

Live-fire testing of the jet's gun system for attacking ground targets and in dogfights against enemy jets faces new delays, Gilmore said. In flights last month, symbols on the helmet display used by pilots to aim air-to-ground attacks were "unusable and unsafe to complete the planned testing."

Joe DellaVedova, spokesman for the Pentagon's F-35 program office, said in an email that "there have only been a couple of flights" where the stability issue "was apparent, and the flight test data is still under review to determine root cause." Additional improvements were incorporated in a software update released in November, and pilot evaluations are planned.

Concerning the development phase's status, DellaVedova said the program office and Lockheed continue "to drive toward the completion of the test program, including solutions" for the issues cited by Gilmore. The program intends to complete all the flight-testing of the most capable software by late 2017, with delivery of the capability to deployed aircraft from late 2017 to spring 2018, he said, although the schedule could slip about three months.

The Navy's version of the plane, the F-35C, also has inadequate wing strength, Gilmore said. Its wingtips aren't strong enough to carry the AIM-9X short-range air-to-air missile, a primary weapon, at some altitudes and airspeeds. Testing on a fix is under way.

"This is a serious deficiency that would have restricted" F-35C flight with the missile, Army Major Roger Cabiness, Gilmore's spokesman, said in an email. The initial defect reports on the structural weaknesses surfaced in 2013 "so the Navy is not just learning about it, but the proposed fixes are just now being implemented," Cabiness said.

In addition, "excessive F-35 vertical oscillations," or shaking, in catapult launches from aircraft carriers must be resolved, Gilmore said.

The Navy has identified the shaking as a "must-fix deficiency'' but "the program waited so long to take action that it is unlikely a solution can be implemented within" the development phase "unless a quick fix is developed soon," Cabiness said.

Rear Adm. Roy Kelley, director of the Navy's F-35C integration office, said in an email that "we are confident that the program will address discrepancies found and aggressively pursue the fixes." Kelley said the development phase "has identified a few F-35C specific discrepancies, to include the load limitations for carriage of AIM-9X, as well as the vertical oscillations during catapult launch."

A potential fix for the wingtip limitations "has already been implemented with the modification of the outer wing panel on F-35C," Kelley said.

