WASHINGTON — Both companies offering clean-sheet designs for the T-X trainer competition have been tight-lipped about their proposals, but glimpses of the proposed aircraft have recently emerged on social media.

These images divulge new details about the designs of Northrop Grumman and Boeing, two of the four prime contractors competing to build the Air Force's next training aircraft.

Photos posted on Twitter on Aug. 19 appear to show Northrop Grumman's T-X prototype during high speed taxi tests in Mojave, Calif. Northrop — which is collaborating with BAE Systems and L-3 on the design — has been reticent to reveal its offering, showing it only once to reporters in 2015.

Northrop Grumman spokeswoman Katherine Thompson confirmed that the images do show the company's trainer prototype, but declined to elaborate on future testing.

"We’ll have more details on that in the months to come," she said.

According to the Federal Aviation Administration registry, the prototype was manufactured by Northrop subsidiary Scaled Composites, which has a history of producing experimental designs. The airplane is powered by the General Electric’s F404-GE-102D.

The prototype shares a number of similarities with the Air Force’s current trainer, the T-38, which was also produced by Northrop, said Richard Aboulafia, an aerospace analyst with the Teal Group. Like the legacy trainer, the company’s T-X design incorporates twin engine inlets and a large vertical tail.

During taxi tests, the aircraft bolts down the runway at high speeds of around 100 to 150 knots but does not take off, he said. Those demonstrations usually pave the way for flight tests, which Northrop has said will occur later this year.

Just days after the Northrop appearance, Boeing quietly launched a wave of new content on its website related to its T-X design, a co-development with Saab, which has been kept under a blanket of secrecy for years.

The site now includes four video "sneak peek" looks at what appear to be computer generated images of the plane from various angles, as well as a pair of new promo images. Notably, the promo images all hint at a "September 2016" rollout. It has widely been expected within industry circles that Boeing and Northrop will both bring their designs to the annual Air Force Association conference, held outside Washington DC starting Sept. 19.

A promo image for the Boeing-Saab T-X trainer design.

Photo Credit: Boeing

Boeing had unveiled just one cockpit promo image of the plane, at last year’s AFA. The new image appears to have a pointier nose than in that preview. However, like Northrop and the other competitors, the plane largely looks like a modernized T-38 and not anything dramatically different in design.

On Twitter Monday, the official Boeing Defense account said more details on the design would be unveiled Sept. 13, the week before AFA begins.

Northrop and the Boeing-Saab team are the only competitors offering clean-sheet designs for the T-X program. Other vendors are modifying existing aircraft in the hopes of paring down risk and cost. Lockheed Martin and KAI have teamed up on the T-50A, a version of the latter vendor’s T-50 operated by South Korea, Indonesia, Philippines and Iraq. Raytheon, Leonardo and CAE are building the T-100, based on Leonardo’s M-346 platform flown by the Italian, Israeli and Singapore air forces.

The Air Force plans to buy a total of 350 T-X trainers to replace its legacy T-38 fleet, with initial operational capability planned for 2024. The service in July released the latest version of the draft request for proposals. A final version is set to be published by the end of the year.

CORRECTION: The article previously classified the T-38 as a single engine trainer. It has two engines.

Aaron Mehta was deputy editor and senior Pentagon correspondent for Defense News, covering policy, strategy and acquisition at the highest levels of the Defense Department and its international partners.

Valerie Insinna is Defense News' air warfare reporter. She previously worked the Navy/congressional beats for Defense Daily, which followed almost three years as a staff writer for National Defense Magazine. Prior to that, she worked as an editorial assistant for the Tokyo Shimbun’s Washington bureau.

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