Boeing Co. landed an order for 100 of its newest and longest 737 Max planes from United Airlines, as another potential customer, Delta Air Lines Inc., opted to take more of a rival Airbus SE jet. The U.S. carriers are the two biggest operators of Boeing’s largest current narrow-body model, the 737-900ER. The third, Indonesia’s Lion Mentari Airlines PT, announced an order for the 737 Max 10 on Monday, hours after Boeing formally launched the latest member of an aircraft family that has flown for a half century. The order from United Continental Holdings Inc. gives the new plane a crucial initial boost and makes the Chicago-based carrier the biggest buyer of the model. United said it was converting 100 previously placed 737 orders to the Max 10, which is designed to haul as many as 230 people on transcontinental flights of about 3,000 nautical miles. “The 737 Max 10 will enable us to continue using larger and more efficient aircraft within our domestic network and better meet the needs of our customers,” United Chief Financial Officer Andrew Levy said in a statement Tuesday during the Paris Air Show. While Boeing hasn’t announced a list price for the Max 10, Lion Mentari’s $6.24 billion commitment for 50 planes implies a value of $124.8 million a plane. That would mean the United order conversion would be worth about $12.5 billion before discounts that are customary for aircraft purchases. United also announced an order for four additional 777-300ER wide-body jetliners from Boeing. Separately, Delta boosted an order for Airbus’s A321 by 10 planes. Boeing is relying on the Max 10 to stem customer defections to the A321neo, which has racked up a considerable order lead since its introduction three years ago.