For the past quarter-century, Boeing (NYSE:BA) and Airbus (NASDAQOTH:EADSY) have competed viciously in all of the biggest segments of the jet market.

The Airbus A320 family squares off against Boeing's 737 family (and previously, its 757 family) in the market for single-aisle jets that mainly fly short-haul routes. Meanwhile, Boeing's 767, 777, and 787 widebody families have faced Airbus' A330 and A350 families (and in prior years, the A300 and A310) in the market for jets with intercontinental range.

Their next big battle looks like it will cover a market segment that barely even exists today. Boeing and Airbus are looking to bridge the gap between single-aisle jets and widebodies in order to capture the "middle of the market." So far, they seem to have very different ideas of how to attack this new "MoM" segment.

Airbus is starting with an advantage

In recent years, the middle of Boeing's product lineup has been a big weak spot. Its largest single-aisle jet (the 737 MAX 9) and smallest widebodies (the 767-300ER and 787-8) have fallen out of favor -- or never became popular in the first place. Meanwhile, Airbus' A321 and A321neo have become extremely popular with airlines.

A Delta Air Lines Airbus A321

Airbus' A321 has become increasingly dominant in its market segment. Image source: Delta Air Lines.

In recent years, Airbus has moved to extend this advantage by offering a longer-range variant of the A321, known as the A321LR. The A321LR, which will be available starting in 2019, is expected to have enough range to fly from the Northeast to many of the major cities in Western Europe.

Boeing plans to introduce a slightly larger single-aisle model (tentatively called the 737 MAX 10X) next month, in an attempt to compete more effectively with the A321neo. But the A321LR is still virtually unopposed for airlines that want to operate transatlantic flights without stepping up to a full-blown widebody. (The 737 MAX 8 can only fly the shortest transatlantic routes.)

Boeing mulls a new plane; how will Airbus respond?

While Boeing hasn't committed to anything yet, it seems increasingly likely that it will launch a new "797" MoM aircraft in the next year or two.

The 797 would be a twin-aisle plane but would have a smaller cargo hold than a typical widebody, thus reducing weight and boosting fuel efficiency. Two different versions would seat 220-260 passengers in a standard multi-class configuration and have a stated range of up to 5,000 nautical miles. (Effective range is typically lower in reality.)

Unfortunately for Boeing, it could easily cost $15 billion to develop the 797 family. Meanwhile, airlines are demanding low acquisition costs, making the business case shaky.

By contrast, Airbus' most likely plan for the MoM segment is to stretch the A321LR to boost its passenger capacity and equip it with a new wing to give it more range. This "A322" would be relatively cheap to design and build, allowing Airbus to undercut Boeing on price by a wide margin. It would probably be almost as large as the smaller 797 variant, but might be limited to 4,500 nautical miles of stated range.

These concepts will appeal to different airlines

One of the most interesting aspects of this looming middle-of-the-market battle is that Boeing and Airbus are approaching it in very different ways. Boeing is leaning toward an all-new twin-aisle design; Airbus is thinking about a single-aisle derivative. As a result, the 797 and A322 could end up appealing to different groups of airlines.

The 797 is likely to be significantly more popular among legacy carriers. Indeed, executives from both Delta Air Lines (NYSE:DAL) and United Continental (NYSE:UAL) have spoken favorably about the concept, despite initial hesitations.

One potentially large advantage for the 797 is extra range. Carriers like Delta and United are perfectly happy with their current options for domestic routes. They want a plane that can replace 767s -- and, to a lesser extent, 757s -- on as many international routes as possible, while potentially making new routes viable by reducing trip costs.

A Delta Air Lines Boeing 767

Delta and United are looking for a plane to replace their aging 767s. Image source: Delta Air Lines.

The difference between 4,500 nautical miles of stated range and 5,000 nautical miles of stated range is significant in terms of the number of routes that would be feasible. (This factor could also convince some low-cost carriers to opt for the 797.)

A second major advantage for the 797 -- from a legacy carrier perspective -- is that it would probably offer a much better customer experience. In recent years, offering flat-bed seats with direct aisle access has become an absolute must for international business class. Delta Air Lines has met this standard for its entire international widebody fleet since 2014. United Continental is on the way there.

However, it's hard to offer direct aisle access for every business class seat on a single-aisle plane (at least without wasting a lot of space). That's a big point in favor of a twin-aisle design like the 797. Economy class passengers would benefit, too. A twin-aisle 797 would have more window and aisle seats, and the cabin would have a roomier feel than a hypothetical A322.

On the other hand, Boeing will never be able to match Airbus on pricing for a 797 vs. A322 battle. Additionally, Airbus would be able to offer commonality with its hugely popular A320 aircraft family. This would make the A322 a sure winner with low-cost carriers, especially as many of them already fly A320-family planes. Legacy carriers also might adopt the A322 for 3,000-4,000 mile routes with light demand and fewer business travelers.

Boeing and Airbus seem to have quite different ideas of what a MoM plane should look like. It will be interesting to see whether either of these concepts becomes a clear winner with airlines, or if Boeing and Airbus fight to a draw in the middle-of-the-market segment.

Adam Levine-Weinberg owns shares of Boeing and Delta Air Lines. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.