SINGAPORE -- Boeing's 737 passenger plane, which has seen deliveries top 5,000 since it entered service 38 years ago, has always been touted by its U.S. makers as the world's most popular commercial jet.

But lately, the plane has been losing business to European rival Airbus' A320, the 737's direct competitor in terms of size, range and listed price. Both single-aisle planes are designed to fly a short-to-medium range and seat a maximum of about 200.