Pentagon tells Congress of new $771 million F-35 cost spike

Lockheed defended the F-35 program via its official Twitter account.

"The F-35 team is focused on reducing costs of the jets and is showing significant improvement in key areas," it said.

But McCain took issue with that phrasing. He responded, "@lockheedmartin To most observers, a $771M cost overrun for 28 F-35s doesn’t qualify as "significant improvement." Taxpayers deserve better."

The F-35 program is the most expensive in Pentagon history, and is being developed for three U.S. military services and eight American allies. But it has a long history of developmental problems that have triggered lengthy schedule delays and pricey cost spikes.

The Pentagon is planning to buy over 2,400 models at a cost of $382 billion -- far more costly than first projected.

Defense experts agree that the program will be on many "cut lists" as Washington attempts to fix its broken finances -- at least until it conquers its remaining technical demons.