Boeing Co is looking to make the first test flight of its 777X twin-aisle jet as soon as late June, people familiar with the matter told Reuters on Wednesday, slightly later than the planemaker had planned, but still on track for putting the plane into airline service in 2020.

Boeing hopes the new jet will extend its grip on the 'mini-jumbo' market, which it shares with rival Airbus SE's 365-seat A350-1000, and perhaps scoop up fresh orders following the demise of Airbus' A380 superjumbo.

The test flight will likely be too late for Boeing to hit its previous target of flying the 777X to the Paris Air Show, which takes place June 17-23, where it had hoped to make a marketing splash in Airbus' backyard.

Boeing declined to comment on its flight schedule for 777X or its plans for the air show. The company has previously said the 777X would fly for the first time in 2019 and will start to be delivered to airline customers in 2020.

Development of the plane has been slowed by the partial U.S. government shutdown earlier this year and assembly delays on its carbon-composite wings and General Electric Co's new GE9X engines.

Boeing has also diverted resources from the program and others to focus on getting its grounded 737 MAX back in the air after crashes in Ethiopia and Indonesia.

The company had been working toward a first flight as early as June 21, effectively overlapping with the air show in Le Bourget, a few kilometers north of Paris, according to two people familiar with the matter. Both people cautioned that the date should be thought of as a window that could stretch into early July.

A third person familiar with the matter said late June was Boeing's current target but added the flight could happen in the United States at the same time as the air show.

The first two 777X flight test aircraft have left Boeing's Seattle-area factory and are in integrated system testing on the ground, while the next two flight test airplanes are in final assembly, Boeing's Chief Executive Dennis Muilenburg told a conference on Wednesday.

"We still expect to achieve entry into service in 2020," Muilenburg said.