Airlines are increasingly concerned that the first deliveries of Boeing's new 777X will arrive late, representatives of both Emirates (EK, Dubai Int'l), the type's launch customer and biggest recipient, and Lufthansa (LH, Frankfurt Int'l), the biggest customer in Europe, have told the Wall Street Journal.

They are drawing up contingency measures to keep older aircraft in their fleets in case the new long-haul, twin-aisle aircraft type fails to show up on schedule.

Boeing could conduct its first 777X test flight in late June, sources close to the matter told Reuters on May 30. This would be too late for the Paris Air Show on June 17-23, where Boeing had previously hoped to make a high-profile marketing splash for the aircraft. But first deliveries are still on track for 2020, the sources said.

Boeing declined to comment to Reuters on its schedule for the 777X or its plans for the Paris event. The company has publicly stuck to its message that the 777X will fly for the first time in 2019 and start to be delivered in 2020.

Tim Clark, chief executive of Emirates, which has one hundred and fifty 777Xs on order, appeared to confirm to the Wall Street Journal on the sidelines of the IATA AGM in Seoul that an inaugural flight would indeed take place after the Paris Air Show. But he added that the 2020 service start date could be delayed by regulators stung by the 737 MAX's certification issues.

Boeing hopes the new aircraft will strengthen its grip on the ‘mini-jumbo’ market, against Airbus’s 365-seat A350-1000. It first unveiled the 777X in March at an employee-only preview rather than a public event, amid the growing controversy surrounding the grounding of the MAX worldwide.