Funding two ventures may raise development costs while also fostering competition and giving NASA an alternative if one vehicle encounters technical difficulties, said Marco Caceres, director of space studies with Teal Group, a Fairfax, Virginia-based consultant.

Congressional opposition to a similar arrangement for the crew contract has waned as the fraying U.S.-Russia relationship focuses attention on NASA's dependence on Soyuz rockets to put astronauts into orbit, Caceres said in a telephone interview.

"The Russians have done NASA a favor in terms of funding," Caceres said.

Aerospace consultant Richard Aboulafia, with the Teal Group, told Bloomberg that the pending deal "is a good endorsement" that the Boeing 737 redesign effort is "poised to achieve some kind of cost advantage."

 That pressures Boeing to cut the wait for its new 737s, which can stretch to years for some airlines, says Teal Group analyst Richard Aboulafia.

The real problem is long-term," Aboulafia says, noting the record-high output for single-aisle airplanes over several years, with no slowdown until both the A320neo and MAX are in production. "Then you'll have Airbus try to exploit a first-mover advantage and Boeing working hard to ramp up faster and catch up. This all sounds like a recipe for overcapacity by the end of the decade."

Joel Johnson, an international-affairs analyst at The Teal Group, says Iran Air "has seven or eight ancient 747s that have been quietly maintained with needed parts by a European carrier over the years, with implicit approval of the U.S."

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.