One of the biggest questions in the aviation world this year has been whether Boeing (BA -1.02%) will build a successor to the Boeing 757. Most of the speculation has revolved around questions of market size and the potential capabilities of a newly designed airplane.

However, there is one factor that hasn't come up much: engineering resources. Boeing's major development programs are scheduled to conclude by the end of the decade. At that point, all of its products will be fairly new. The one thing left for its engineers to do would be to design a new airplane to fill one of the gaps -- and the obvious choice would be a 757 replacement.

Why the 757 is special
The 757 is unique among single-aisle aircraft in its ability to fly routes up to about 4,000 miles: enough to go from New York to Western Europe. There are two big advantages to being able to fly a long-haul route with a single-aisle plane rather than a widebody.

Several airlines use the Boeing 757 to fly from the East Coast to Europe.

First, single-aisle planes weigh a lot less on a per-passenger basis. That typically means better fuel efficiency and lower landing fees. Second, they carry fewer passengers, and can therefore economically serve lower-traffic routes -- or can fly more frequently on busy routes.

The 757 is getting old; it was designed in the late 1970s and early 1980s. Newer planes are far more fuel-efficient, negating one of the main potential advantages of a long-range narrowbody. The 757 could also be deployed on far more routes if it had just a little more range.

Boeing's rival makes its move
Airbus (EADSY 0.30%) is marketing a long-range version of its upcoming A321neo to bring the long-range narrowbody into the 21st century. It's about 10 seats smaller than the 757-200, but will have roughly similar range. Airbus claims that the long-range version of its A321neo will consume 30% less fuel than a 757.

The biggest trade-off in Airbus' design is that there is limited room for cargo and baggage in the hold. The fuel-efficiency gains should easily exceed any losses in cargo revenue, though.

Boeing initially dismissed the market potential of Airbus' new plane, arguing that there were only a handful of 757s flying routes outside the range of a 737 MAX 9. But airline interest in a plane with somewhat more capacity and range has been growing. As a result, Boeing has been coming around to the idea that there is a need for a plane that is in-between the smaller 737 and the larger 787 in both size and range.

Is there a business case?
Boeing has pointed out that, while the 757 was a successful program for its day, replacement demand is too small to justify a new design. Boeing only built 1,050 757s over a span of more than two decades. That's a pittance, considering that it now churns out more than 500 737s every year. Furthermore, many 757s have already been replaced with the Boeing 737-900ER or the Airbus A321.

But a potential replacement could potentially create a large new market. A larger narrowbody could offer even lower unit costs than the 737 MAX 9 due to its higher passenger capacity. That would make it a good fit for busy routes between capacity-constrained airports. Meanwhile, extra range would make new routes in the 3,500-5,000 mile range possible.

There could be demand for 3,000-4,000 of this type of aircraft, according to analyst Richard Aboulafia of Teal Group. The big issue is cost. Airlines don't want to pay widebody prices for a "middle-of-the-market" plane like this. That means Boeing can't plan to recoup high development costs through high prices.

Boeing will figure this out
If Boeing had other priorities in its development pipeline, the difficulty of designing a new aircraft that meets airlines' needs at an attractive price might be a deal-breaker. But all of Boeing's development programs are nearing their ends.

The 737 MAX will enter service in late 2017. The stretched 787-10 variant of the Dreamliner will follow a year later. And the biggest project underway, the 777X, will be ready for its first delivery by 2020.

All of Boeing's new aircraft projects are scheduled to wrap up in the next five years. Photo: Boeing.

Given that Boeing doesn't contemplate bringing a brand-new 737 replacement to market until 2030, it's not clear what Boeing's engineers would be doing in the 2020-2025 period if Boeing doesn't enter a new market segment. (Boeing's shrinking defense business isn't likely to soak up much in the way of engineering resources.)

A 757 replacement is the most logical project that could be scheduled during this window. It can potentially be designed in a way that many of its parts and systems could be reused for the 737 replacement due to hit the market around 2030.

This would represent a clever way to minimize development costs, keeping the plane affordable. It would also help Boeing get a jump on development of its 737 replacement, even if it didn't have all the technology ready to complete that plane until 2030.

There certainly seems to be a market for a larger, longer-range narrowbody. Boeing's biggest challenge is the need to develop it cheaply.

Fortunately, its engineers will have plenty of time on their hands to figure out how to do this. Generally speaking, Boeing shouldn't decide whether or not to design new planes based on whether it has engineers available to work on them. But in this case, it could tip the scale in favor of going ahead with the project.