The first flight of China’s Comac C919 airliner is behind schedule and delivery could be pushed back by up to two years, dealing a blow to its push to challenge Airbus and Boeing. The narrow-body aircraft, which will be able to carry 156-168 passengers and aims to compete with the Airbus A320 and Boeing 737, was originally scheduled to fly by end-2015 but two people familiar with the matter said it would be delayed to the first half of 2016. Delivery of the first plane, scheduled for 2018, is also likely to slip, perhaps to 2020.