The head of Boeing’s 787 program said Monday that the company is confident it can lighten the hot-selling plane by 2 1/2 tons — enough to fulfill promises that it will be much more fuel-efficient than any similar commercial jet flying today.

Engineers are looking at ways to trim weight from parts throughout the plane, in some cases switching from aluminum to titanium, Mike Bair, general manager of the program, said in a quarterly conference call on the plane’s development.

“We know exactly where we have to go to take weight out of the airplane,’’ Bair said.

Boeing Chief Executive Jim McNerney recently called the 787’s weight a “dogged issue’’ that would force the aerospace giant to boost research and development spending on the plane by $300 million this year and in 2007. That’s in addition to the roughly $335 million in extra R&D spending Boeing committed to the 787 in the second quarter of this year.

Boeing has estimated it needs to shed roughly 5,000 pounds from the 787. Once that happens, Bair said, the company will be able to meet the commitments it’s made to the 36 customers that have ordered the plane so far.

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