WASHINGTON — Rep. Jackie Speier is demanding answers after the US Air Force reportedly botched estimates for the 10-year cost of its next-generation bomber.

The California Democrat sent a letter to Air Force Secretary Deborah James on Monday morning decrying "massive discrepancies" between this year and last year in 10-year cost estimates for the Long Range Strike Bomber (LRS-B), and demanding an explanation by Sept. 30.

Last year, the Air Force estimated costs for the LRS-B from fiscal years 2015 to FY-24 2024 at $33.1 billion. This year, the service pegged costs for FY '16 to FY '25, a similar 10-year period, at $58.2 billion. This is a 76 percent increase, Speier points out in the letter — a change the ranking member of the House Armed Services Committee's subcommittee on oversight and investigations calls "alarming."

"This sudden 76 percent increase in estimated cost is alarming, because it raises questions about the management of a crucial program that lacks transparency, on which we cannot afford serious cost overruns, development errors, and reduced production numbers that would deprive the United States of one of its core military capabilities," Speier wrote.

The Air Force has now said the true cost estimate for both 10-year periods should be $41.7 billion, service spokesman Ed Gulick said Monday.

Although the Air Force maintains program costs remain stable, the discrepancy suggests the Pentagon's projected cost of the aircraft — $500 million per plane — may be unreliable, Speier wrote.

"The Defense Department has promised that the LRS-B would be produced at a fixed price of $500 million per plane, but these reports suggest we should be concerned about the reliability of that promise," Speier wrote.

Speier demands the Air Force explain the root cause of the discrepancy and provide additional information about how the cost estimate is calculated by Sept. 30. Speier also dings the Air Force for a lack of transparency, pointedly asking what steps the service is taking to ensure Congress is aware when such large discrepancies are identified in future.

Comparing the LRS-B to the B-2 program, which faced huge cost overruns after being developed entirely behind closed doors, Speier also asks the Air Force to provide answers on how many details about the new program will be declassified.

The Air Force is working to ensure the recent mistakes are corrected, and that reports in future years are accurate, Gulick said.

This is not the first time Speier has called for stricter oversight of the Air Force's secretive bomber program. During the House Armed Services Committee's markup of the 2016 National Defense Authorization Act (NDAA), Speier supported a mark directing the comptroller general to review the service's acquisition strategy for the LRS-B.

The language, which the House adopted in its final version of the bill, orders a comparison of the bomber program's technological maturation versus other service acquisition programs at this point in their development. It would also include an examination of the overall acquisition strategy, technology design, development and testing status, cost and schedule implications, and expected technological performance.

As part of the study, the language directs the Air Force to "ensure timely access to the necessary program information including, but not limited to, cost and budget information, detailed schedules, contractor data, program management reports, decision briefings, risk and technology readiness assessments, and technical performance measures."

This is notable, given the Air Force's reticence to discuss any details of the program.

The House and Senate are still in conference discussions over the NDAA, with a final bill expected later this summer.