WASHINGTON — Senators are seeking cost-cutting opportunities in the Pentagon’s $400 billion program for the next-generation F-35, a fighter jet with a troubled testing record that military leaders say America cannot afford not to build.

Chairing the hearing, Sen. Dick Durbin on Wednesday compared the F-35’s history to a textbook on how not to develop a plane.

The Illinois Democrat asked military leaders to justify a decade of expanding costs.

They stressed that costs were now decreasing.

Pentagon acquisitions chief Frank Kendall said the plane is 90 percent developed. Testing is almost half-done.

Kendall said no one is considering stopping the program.

The F-35 would replace Cold War-era aircraft including the F/A-18 Hornet and AV-8B Harrier. It’s the Pentagon’s priciest weapons program ever. Estimates suggest costs could reach $1 trillion.