Boeing Co must sell more 777 and 787 jetliners to keep production plans on track, despite a $13.8 billion order that landed earlier on Thursday, its chief executive said. The order from high-profile customer Singapore Airlines is a strong endorsement of both planes, and could lift sales in a sluggish market if other carriers follow Singapore Air's lead, analysts said. Sales of Boeing's highly profitable 777s have slowed so much that Boeing is cutting production 40 percent this year to cope as it begins switching to the successor 777X model that Singapore Airlines ordered.