* Marketing chief says new plane needed to fill mid-market gap
* Industry sources see family of two twin-aisle jets
* Project set to create industrial jigsaw for 737 replacement
By Tim Hepher
CANCUN, Mexico, June 7 (Reuters) - Boeing has looked at options "from mild to wild" for the design of a proposed mid-market jet, a senior executive said, hinting at a breakthrough that industry sources say will create building blocks for future models.
Marketing Vice President Randy Tinseth said Boeing would leapfrog reported plans by Airbus to update its hot-selling A321neo, as Boeing eyes a gap between narrow-body jets and long-haul aircraft for a potential new mid-market airplane.
"We have looked at the mild and we have looked at the wild and I can tell you we know that if you are going to address that market, you need a new airplane," Tinseth told Reuters after a two-day meeting of airline leaders in Mexico.
Industry sources have said the mid-market development is pivotal for Boeing since it will spawn the industrial jigsaw, systems and cockpits likely to be used for the next plane after that, a three-aircraft replacement of Boeing's 737 cash cow.
Getting the "production system" right now would partially allow Boeing to develop the next jet, which is expected to revolve around a model carrying 180 passengers, as an industrial spin-off of the mid-market one, albeit with major differences.
This would result in significant cost savings and avoid repeating a patchwork of different production architectures.
Two further derivatives could extend that post-737 jet family to 160-210 seats, based on current market forecasts.
Boeing has not yet talked about its plans beyond the mid-market plane, which is expected to enter service by 2025.
Boeing officials declined comment on the long-term options or specific details of the mid-market project, which one leasing company has dubbed "797".
For the mid-market jet, industry sources have said Boeing is settling on a family of two wide-body aircraft.
These would effectively combine a twin-aisle cabin sitting on top of the reduced belly space of a single-aisle jet.
The aim is to reduce wind resistance or drag and therefore operating costs.
However, it involves a risky gamble that airlines will not need to carry much paid cargo on the routes for which the airplane is designed, delegates at the airlines meeting in Cancun said.
The two mid-market models, designed to carry about 220-260 passengers over 3,500 to 5,000 nautical miles (6,400-9,260 km), will also have a wing resembling the distinctive stiletto design of the 787 Dreamliner but with significant internal differences.
Seen from the front, the outline of traditional metal airplane fuselages is usually closer to a true circle.
That allows pressurised air inside the cabin to push out uniformly in all directions, easing loads and removing the need for heavy strengthening materials.
That well-tested concept is as old as the steam engine.
Carbon composites allow manufacturers to make complex pieces in one shape and are well suited to the more elliptical design that Boeing has in mind for the new mid-market fuselage.
However, composites are more expensive to produce.
Reuters reported last month that the new aircraft could be built using cheaper and faster new production techniques without costly pressurised ovens, or autoclaves.
That technology was used to weave the carbon wings of Russia's new MS-21 jet, which first flew last month.
Airbus this week played down a project called A321neo-plus-plus in response to the Boeing mid-market jet, first reported by Reuters, and said it was always reviewing options. (Additional reporting by Victoria Bryan; Editing by Susan Fenton)