SEATTLE, March 27 - The Boeing Company has committed to build a longer version of its fuel-efficient 787 jet to meet demand from a number of airlines, a Boeing executive said on Monday.

"It"s not a matter of if, it"s a matter of when, and finalizing some details around configuration," Michael B. Bair, vice president and general manager for the 787 program, said.

The 787-10 will carry about 300 passengers, about 50 more than the largest of three models the company had previously said it would build.

While some details have yet to be decided, such as the precise seating capacity and how far the plane will fly, Mr. Bair said Boeing expected to begin delivering the plane in the final quarter of 2012.

He said "about a dozen" carriers had expressed interest in a stretch 787, but the only one he mentioned by name was Emirates Airlines, a carrier based in the United Arab Emirates. Qantas Airways of Australia has also said it would be interested in a larger 787 if Boeing built it.

Initially, Boeing had resisted the call for a larger 787, fearing it might steal sales from an extended-range version of its 777. Then more airlines started showing interest in it.

To date, Boeing has received 298 firm orders and 88 commitments from a total of 28 airlines for 787 models.