The decision on who will build the Air Force's next-generation jet trainer is not expected until later this year but pressure may have already eased on Boeing to score a must win against its rival Lockheed Martin.

The two defense giants are the likely front-runners in the roughly $16 billion competition known as the T-X Trainer. The winning team will get an order for 350 jets to replace the Air Force's aging T-38 trainers that have been around since the 1960s.

"It would be an important win for Boeing given that it does not have the F-35 and it didn't win the bomber program for the B-21," said Jonathan Root, an analyst at Moody's in New York.

Last year, Boeing's tactical aircraft division suffered a setback when the government rejected the company's protest of Northrop Grumman winning the B-21, an advanced long-range strike bomber deal valued at more than $80 billion.

Yet some analysts who earlier this year saw the competition as a "must-win" for Boeing now believe the pressure may have moderated since the Chicago-based defense contractor may be close to securing more orders on its F/A-18 fighters.

"The pressure has been alleviated some because it looks like the F-18 is going to get a little bit of a resurgence here," said Richard Safran, a defense analyst at Buckingham Research in New York. He noted that recent reports have indicated that the government may do an additional buy of an upgraded F/A-18 so that would provide a boost for Boeing's tactical aircraft business.

Even so, Boeing and its bid partner, Swedish aerospace manufacturer Saab, may need to absorb the development costs on the company's so-called clean-sheet version of the trainer aircraft. In contrast, Lockheed's entry in the competition is the T-50A in partnership with South Korea's Korea Aerospace Industries and uses a platform based off the existing T-50 Golden Eagle, a legacy plane introduced more than a decade ago.

Overall, there are at least five separate bids in the T-X Trainer competition.

Analysts estimate the Boeing team already has spent $1 billion to $3 billion on development costs associated with its new trainer.

"They still have the problem of making all those upfront development costs disappear in their bid price — and that's challenging," said Richard Aboulafia, vice president of analysis at Virginia-based industry consultancy Teal Group. He said Boeing's price on the plane could end up being higher "unless they just want to eat that."

Aboulafia calls the T-X trainer contract "Lockheed Martin's to lose but not a slam dunk." That said, he believes Boeing could prevail if it puts in "an extremely aggressive bid."

Lockheed, a Bethesda, Maryland-based defense contractor, touted its entry for the T-X contract on the company's website by noting the single-engine, two-seat trainer has registered more than 142,000 flight hours and "is ready to deliver on day one."

The T-50 Golden Eagle was previously procured by several international air forces, including South Korea, the Philippines and Thailand, among others. If Lockheed wins the trainer contract, it is expected to build the aircraft in South Carolina.

"We're excited to offer the U.S. Air Force a low-risk proposal that meets all their needs for advanced pilot training, and can help them achieve initial operational capability earlier," Mike Griswold, Lockheed Martin T-50A's business development director, said in a statement to CNBC.

The Pentagon started accepting final request for proposals in late December for the T-X Trainer and the formal deadline was last week for submissions. The Air Force is expected to use the new trainer for pilot training for the F-35 as well as the F-16 and F-22 fighter aircraft.

Boeing said on its website that its trainer jet features a single engine, twin tails, stadium seating and claims its entry is "more affordable and flexible" when compared against older, existing planes.

Boeing has already built at least two of the jets and said it is prepared to build more.

"We're confident that our all-new, purpose-built Boeing T-X advanced pilot training system — which includes the aircraft, ground-based training and support designed together from the ground up — is the right choice for the U.S. Air Force," Boeing said in a statement.

Analysts say the Boeing trainer jet, which closely resembles the Super Hornet, is considered heavier and also might contribute to it costing more to produce than the Lockheed entry.

Boeing's trainer jet had its inaugural first flight in December.

Boeing could make a case to the Air Force that it should get the trainer jet contract because it has an advantage in manufacturing, or it could claim the plane is easier to assemble, industry experts suggest.

Then again, Boeing also might be willing to accept a lower margin with the U.S. government if it believes a high-performance trainer might have potential in the export market with allies. In addition to selling 350 training jets to the U.S. Air Force, Aboulafia said the allies could potentially buy another 100 to 150 planes.

Other T-X Trainer bidders include a unit of Italy's Leonardo in building its T-100 trainer at a factory in Alabama. Stavatti Aerospace proposed its Javelin T-X as a trainer, a jet originally designed as a civilian sport aircraft. Sierra Nevada Corp. and Turkish Aerospace Industries also are expressed interest this year in teaming in a joint venture to offer the Air Force an advanced trainer.

Stavatti CEO Christopher Beskar said in an email response that the Javelin T-X entry is "a re-imagining of the original ATG Javelin aircraft that first flew in 2005." He said the aircraft's flyaway cost is $10 million per jet and the executive added that the company submitted a second bid offering for the SM-47 T-X, a jet costing $20 million each.

Leonardo, Sierra Nevada and TAI didn't respond to requests for comment.

In January, Raytheon ended its partnership with Leonardo for the T-X Trainer program. Northrop Grumman, maker of the legacy T-38 jet currently used as a trainer, had been partnered with U.K.-based BAE Systems but pulled out of the competition in February.

Finally, Textron also had earlier been expected to participate with its Scorpion jet but ultimately exited the competition.