Speaking at the company's meeting with investors in Seattle, McNerney said he expects technological breakthroughs that will enable a redesigned version that would be a big improvement over the existing design.
A redesigned plane would take longer to bring to market but would provide greater fuel efficiency.
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Boeing has said its customers are calling for an all-new plane. But rival Airbus <EAD.PA> has said it would put a new engine in its competing A320 aircraft.
"We are going to retain the ability to re-engine if this new airplane doesn't come together over the next nine months or so as we think it will," McNerney told analysts at the webcast meeting.
McNerney, who expects a redesign, said Boeing is evaluating how big the next 737 should be. "As we think about it right now, it may be modestly bigger."
(Reporting by Kyle Peterson; Editing by Tim Dobbyn)