Boeing Co. is growing more optimistic about airplane orders, increasing its long-term forecast on the expectation that demand for air travel will expand around the globe, led by China and other developing countries. Airlines will order new planes valued at $5.9 trillion over the next two decades, Boeing said Monday, up 4.1 percent from a year earlier. That would represent a total of 39,620 jetliners across the industry. The rise of discount carriers and growth in developing countries will fuel aircraft purchases despite recent economic turmoil from Brexit and a commodities slump, Boeing said. About $3 trillion of the projected orders will come from single-aisle aircraft such as Boeing’s 737-800 and Airbus Group SE’s A320, according to the U.S. planemaker. “The aviation sector will continue to see long-term growth with the commercial fleet doubling in size,” Randy Tinseth, vice president of marketing for Boeing’s commercial airplanes division, said in a statement. Demand Shift About 28,140 new airplanes will be needed in the narrow-body segment, Boeing said. The company also expects to see a wave of wide-body orders next decade as airlines replace the twin-aisle jets that ferry travelers across the Pacific and on other long-range routes. Demand is likely to shift from the largest long-haul jets, such as Boeing’s 747 jetliner, to smaller aircraft. The Chicago-based planemaker expects sales to be concentrated in the 200- to 300-seat part of the market, where the company sees demand for 5,100 wide-body jets. Boeing is studying a new family of aircraft targeting much of this segment, which it has dubbed the middle of the market. The manufacturer lowered a forecast of medium wide-body orders by 1.4 percent to 3,470 from a year earlier, and trimmed anticipated sales of very large aircraft such as the Airbus A380 superjumbo by 1.9 percent to 530 sales.