Boeing Co (BA.N) said on Wednesday it is not studying resurrecting its out-of-production 757 jetliner with new engines to plug a gap in its jetliner lineup between its largest single-aisle plane and its smallest widebody models.

Boeing has studied resuming production of the single-aisle 757 "a couple of times," Randy Tinseth, vice president of marketing for Boeing Commercial Airplanes, said at an industry conference organized by the Pacific Northwest Aerospace Alliance.

"That airplane had a very unique production system. It was relatively expensive to build," he said. "The business case is not going to close."

The statement was Boeing's clearest indication that it was not considering reviving its old jet. It has said previously that it was not planning to build a rival to a long-range version of Airbus' forthcoming single-aisle plane, the A321neo, which Boeing says is only catching up to the capability of its 737 MAX.

Boeing stopped building the 757 in 2005, after delivering 1,049 of the planes, known for their powerful engines that allow them to takeoff on short runways and carry 200 passengers in a typical two-class configuration.

Tinseth said there are about 550 passenger 757s still in operation, and that airlines still using them have mostly ordered Airbus A321s or 737 MAX-9 planes to replace them.

Boeing is asking airlines what capability they want in a 757 replacement, filling the space between the 737 and Airbus A320 and the twin-aisle 787 and A330.

"They want an airplane that's bigger than today's 757 and flies farther, probably 20 percent farther," he said.

"It's going to take some time" to firmly configure the new jet, he added.

Boeing Chief Executive Officer Jim McNerney said in November that the company would bring out a 737 MAX replacement around 2030, with a new fuselage, wings and engines. That plane will also address the gap left by the 757.