Boeing and Saab are among contenders for a deal to replace the aging T-38 trainer in a competition known as T-X.

Saab Chief Executive Hakan Buskhe said he was "very happy" with the two-year-old partnership with Boeing and that he was not worried about the cost of investing in their new design, even though it was "significant".

"It is so tempting that you can't stay out and ... I think we will win. That is my view," Buskhe said of the competition, speaking to Reuters at a London defense exhibition.

"If we don't win, we have learned a lot of other things and we have shown it is possible to work together and have created a culture between Boeing and Saab (such) that we could do more things together."

Despite being rival fighter manufacturers, Boeing and Saab already co-operate in adapting an air weapon for ground forces.

In 2013, they agreed to develop a new plane to try to win the U.S. order for 350 trainers. The U.S. Air Force plans to launch that competition formally in 2017 and analysts say it could eventually buy up to 600 planes.

The design of the Boeing-Saab proposal has been one of the industry's closest secrets, but Boeing offered a glimpse at an Air Force Association event in Washington this week.

A teaser graphic showed a long and slender nose section and partial cockpit, with no engine inlets or wings in sight.

Experts said that indicated the aircraft may be larger and more ambitious than traditional trainers such as Britain's popular Hawk, whose wings and engines sit close behind the pilot.

Northrop Grumman Corp, builder of the current T-38, had originally teamed up with Britain's BAE Systems to offer the Hawk, but tightened the contest earlier this year by announcing it had switched to an all-new model.

Also offering a clean-sheet design is Textron Inc.

Lockheed Martin has partnered with Korean Aerospace Industries to offer the South Korean T-50.

Italy's Finmeccanica, recently abandoned by U.S. partner General Dynamics in a bid to offer a jet based on the Italian M-346 trainer, is looking for a new U.S. partner, executives said at the DSEI defense event in London.

Defense News quoted U.S. Air Force officials as saying U.S. contractor Raytheon was discussing the possibility of taking over as the lead partner for Finmeccanica's aircraft. Raytheon officials were not immediately available to comment.

(Reporting by Tim Hepher; Editing by Catherine Evans)

By Tim Hepher