WASHINGTON — Boeing and Saab have managed to keep the design for the Air Force's T-X trainer competition under wraps, but there are signs the two companies may be nearing a milestone for the program.

On Monday, aviation enthusiasts in Sweden noticed the arrival of an Il-76, operated by Volga-Dnepr Airlines LLC, near the city of Norrköping, where a large piece of cargo was loaded onto the plane. From there, the large cargo plane took off for Reykjavik, Iceland — and then took an overnight flight to St. Louis, Missouri.

So what was the cargo loaded at Norrköping? Local radio station P4 Östergötland claim that the cargo aboard the plane is related to the T-X program, notable given the final destination for the cargo; St. Louis is home to Boeing's fighter production line and is widely expected to be where any T-X work gets done for the company. That report set off a frenzy for enthusiasts in the region — and led to local media arriving to snap pictures.

The pictures show a large package covered with a white, protective covering. It is hard to tell because of the cover but the size of the package seems to indicate some sort of fuselage, minus the wings and nose of the jet. If that is correct, those other parts may be shipped independently, or it may be that Boeing is producing those in the US — which would be news, as the exact production agreement between Saab and Boeing has remained largely in the shadows.

A spokeswoman for Boeing would neither confirm nor deny the transport is related to the T-X program, while a Saab spokesperson declined to comment on company transfers as a matter of policy.

Boeing has said that the jet will make its first flight this year, and industry speculation is that the Boeing-Saab team may unveil the design fully at September's Air Force Association conference, held just outside Washington, D.C.

The T-X competition will replace the service's aging T-38 trainer design with 350 new planes, high-end enough to train pilots on the F-22 and F-35 fighters. The Air Force expects to issue a request for proposal at the end of this year, with the trainer going fully operational in 2024.

In addition to the Boeing-Saab team, Lockheed Martin and KAI are offering the T-50A while Raytheon, Leonardo and CAE are teamed on the T-100, based on the M-346 design. Northrop Grumman, BAE Systems and L-3 are collaborating on a clean-sheet design, as well.

Twitter: @AaronMehta