The Secretary of the Air Force said Friday that the first flight of Boeing Co.’s (NYSE: BA) new tanker, the KC-46A, is likely to be delayed by several months. The first flight of the plane is still scheduled for next month, but the Air Force general who is in charge of the program said last week that he was “not comfortable” with committing to the April date.

Secretary Deborah Lee James told Defense News on Friday, “My best belief, at this point, is it will be a several-month delay. So hopefully summertime is when it would occur.”

Under the terms of its contract with the Air Force, Boeing must deliver the first 18 KC-46As by 2017. Boeing officials say that their focus is on meeting that contractual obligation and not the interim target dates. In his comments on Tuesday, program executive officer General Duke Richardson said that the entire six-month margin built into the schedule has been used and that a delay in getting the first test flight in the air could have an impact on getting air worthiness certificates in time to meet delivery commitments. Richardson said, “… I’m not looking for the perfect airplane. I’m looking for a safe airplane so I can get it up and start collecting the air worthiness data.”

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Secretary James told Defense News that Boeing has submitted for Air Force review a new master schedule for the program. She’s worried:

The worrying news is that underneath those contractual and milestone requirements, there are a whole lot of other milestones. This is the internal plan for how do you get from here to there to meet the milestones. That’s where there have been challenges and slippages and so forth, so that is the worrying part.

The first test flight of a 767-2C, a modified version of Boeing’s 767-200ER commercial plane and the base on which the KC-46A is built, was delayed from June of 2014 to December.

The KC-46A program also faces an unexpected hurdle that was thrown up by Congress related to replacing the venerable Northrop Grumman-built A-10 “Warthog” close-air support plane. In order to transition maintenance and support smoothly to a new plane, the Air Force wants to retire the A-10 and move the maintenance crews to Lockheed Martin’s replacement F-35. The Air Force has a similar plan for the KC-46A. Congress added language to the National Defense Authorization Act passed in December prohibiting any fiscal year 2015 funds from being used to “transfer, divest, or prepare to divest any KC-10 aircraft.”

Ultimately Boeing is under contract to replace the majority of the Air Force’s tanker fleet with 179 KC-46As by 2028. The total value of the contract is about $52 billion.

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