In the longer term, Mr. Tinseth told a conference hosted by the International Society of Transport Aircraft Trading that customer feedback is pushing Boeing to evaluate a new product that fits between today’s single-aisle 737 and larger long-range 787. He said airlines and lessors are interested in a jetliner that can fly about 4,800 nautical miles and is larger than its 180 to 240-seat 757, which the company discontinued a decade ago.