ST. LOUIS — Boeing is bringing the first tease of its T-X trainer design to next week's Air Force Association conference outside of DC Washington — but only a select few will get to see it.

The company is bringing a T-X demonstrator trailer to the conference, held annually in National Harbor, Maryland. Boeing is teamed partnering with Saab on the design.

It The aerospace company will also be briefing attendees on a new configuration of the F-15, which ups the number of air-to-air weapons on the popular jet to 16.

The T-X trailer will be invite-only, and a Boeing official said there are currently no plans to display the design through models or artwork at the company's booth — meaning the specifications of the plane remain a closely-guarded secret.

One person who ranked a visit? Secretary of Defense Ash Carter, who visited the trailer and was briefed on the program during a visit to Boeing's facilities in St. Louis.

Little is known about the design, despite Boeing having been the first company to announce plans to do a "clean sheet" trainer made specifically for the US Air Force's requirements.

Program officials have specified that the design will not be a modification of Saab's Gripen fighter. Boeing has also hinted to reporters that a first flight of the trainer will happen before the end of the year, but have not confirmed that on the record.

Stu Voboril, Ddirector of Sstrategy & Pprogram Ddevelopment for Boeing's Phantom Works division, told reporters Wednesday that the partnership with Saab was "phenomenal."

The T-X competition, which will replace the service's aging T-38 trainers with 350 new aircraft, has drawn serious interest from industry. In addition to the Boeing-Saab team, Lockheed Martin is teamed with KIA to offer the T-50, Northrop Grumman is offering a clean-sheet design, and the Textron AirLand Scorpion is expected to compete.

General Dynamics was acting as the prime contractor and integrator for the T-100, a modified version of the M-346 trainer sold by Italian firm Alenia Aermacchi, until March when it withdrew from that partnership. Alenia is still searching for a US prime to joint with to stay in the competition.

Boeing will also use the AFA conference to market a new configuration of its F-15 fighter.

The new design features 16 air-to-air weapons, doubling the number currently available on the jet. To accommodate the additional weaponry, the new design alters the location of the fuel tanks.

Marketing a new configuration for the F-15 makes sense, as the line is expected to expire before the end of the decade. More details are expected to be available at the show, held Sept. 14-16.

Secretary Carter also visited the new design, which was on display at the facility.

Twitter: @AaronMehta