Richard Aboulafia, an analyst at the Teal Group, said: "Delta's established a rich pattern for buying older planes. The only surprise here is that they actually went ahead with their order. They were the least enthusiastic about new-generation airplanes."

Delta declined to say how much it would pay for the planes, which have a list price of more than $13 billion. But a single bulk order effectively guarantees it will receive a steep discount.

"You get half off just for showing up," Mr. Aboulafia said.

"The 350 is a very serious threat and it's very well executed on," said Richard Aboulafia, aerospace analyst for the Teal Group, an analyst firm outside Washington, D.C. "Boeing was dragging its heels on the 777X and the 787-10, the A350 is a far bigger success than it would have had Boeing been quicker."

He added that Boeing's strained relations with its two largest unions is a potential liability, as the company develops the design and manufacture of the 777X.

"Bad workforce relations are a risk," he said. "It doesn't guarantee that you're not going to execute as well as Airbus did on the A350, but it adds risk. Airbus has better labor relations."

"Boeing is at a disadvantage in terms of next generation narrow body timing because Airbus is doing a great job with the A320neo," said Richard Aboulafia, aerospace consultant for Teal Group.

First delivery of the A320neo is scheduled for the fourth quarter of 2015 to Qatar, while first delivery of the 737MAX is scheduled for the third quarter of 2017 to Southwest (LUV) .

"Airbus is going to get there the better part of two years earlier than Boeing," Aboulafia said. "The best thing Boeing can do is to catch up with them as soon as possible. In this high-fuel-cost environment, it's a game of availability; it's all about timing."

But critics like Richard Aboulafia, an aerospace analyst at the Teal Group, an aviation-consulting firm in Fairfax, Va., say the main problem is more fundamental: Airbus made the wrong prediction about travel preferences.

He said people would rather take direct flights on smaller airplanes than get on big ones — no matter their feats of engineering — that make connections through huge hubs. "It's a commercial disaster," Aboulafia says. "Every conceivably bad idea that anyone's ever had about the aviation industry is embodied in this airplane."

Richard Aboulafia, vice president of analysis at the Teal Group, believes current narrowbody production rates are sustainable today, "but going up further can be a real problem. You can go up to 50 or 60 [aircraft per month], but if the slightest thing happens you are in real trouble." He cautions that "the idea to go up further is a great way to engineer overcapacity."

Both Airbus and Boeing have already decided to move production rates up for the narrowbody models—Airbus is boosting the A320 line to 46 aircraft per month from 42, and Boeing is moving 737 production to 47 per month from 38 and is pondering 52. Airbus is also looking at rates higher than 50 when the transition to the A320neo is completed.

Aboulafia also is concerned that some emerging players such as Lion Air or Norwegian may not fulfill their promises. Both airlines have large orders for new narrowbodies with Airbus and Boeing. From a macroeconomic point of view, this is worrisome, he says.

To an extent, the situation for manufacturers is as good as it is because of low interest rates (which encourage investment) and the high cost of fuel. These factors can change, and if Airbus and Boeing are unlucky, fuel will become cheaper and interest rates will rise in parallel. Therefore: "We are taking it too far," Aboulafia contends.