Defense Secretary Robert Gates has put the U.S. Marine Corps’ troubled F-35B short-takeoff-and-vertical landing (Stovl) Joint Strike Fighter on “probation,” while endorsing the U.S. Air Force’s long-coveted new bomber program.

The F-35A and F-35C models emerged unscathed from Gates’ review. However, the F-35B “is experiencing significant testing problems,” Gates said at the Pentagon Jan. 6.

Implying that problems are more serious than previously reported, he adds that “these issues may lead to a redesign of the aircraft’s structure and propulsion — changes that could add yet more weight and more cost to an aircraft that has little capacity to absorb more of either.”

The JSF test program will be restructured so that testing of the F-35A and F-35C runs ahead of the B model, rather than the other way around. If the B model cannot be “fixed or gotten back on track” in two years, “I believe it should be canceled,” Gates says.

Gates’ comments came during a press conference announcing a series of budget efficiencies designed to cut or redirect more than $150 billion from current Defense Department spending over the next five years.

Delays to F-35B testing so far — fewer than a dozen vertical landings have been logged since March 2010 — have been publicly attributed to a problem with the auxiliary engine inlet door, and individually minor issues with components such as cooling fans.

More details of changes to the JSF program also emerged, including another delay in the completion of systems development and demonstration (SDD) and a cut-down production ramp. SDD is now delayed to early 2016, versus mid-2015 as planned in the restructuring of the program early last year. SDD finishes with the conclusion of development testing and precedes initial operational testing and evaluation, so the move likely will push initial operational capability (IOC) into 2017. (The individual services are assessing their IOC dates.) This will cost an additional $4.6 billion to the program.

The Fiscal 2012 JSF buy — low-rate initial production (LRIP) Lot V — will be held at 32 aircraft, both to reduce concurrency and because “the final assembly process at Fort Worth is still maturing,” Gates says. Deliveries at this point are late by multiple months.

In Fiscal 2013 and later, deliveries will ramp up by a factor of roughly 1.5 per year, for a total of 325 aircraft through LRIP IX (on contract in 2016 and delivered by 2018) versus 449 in the previous plan.

The LRIP IV contract, just signed, will be changed to eliminate all but three Stovl aircraft. The U.S. will buy only six Stovl aircraft in each of the next two LRIP Lots (V and VI), regarded as the minimum needed to sustain the supplier base and unique skills.

Gates indicated in response to questions that a last-ditch appeal by Marine Commandant Gen. James Amos and his predecessor may have saved the B-model from outright cancellation. Gates said the commandants made a convincing argument for more time to fix the program.

The Navy also plans to acquire more Super Hornets and extend the structural life of 150 “classic” Hornets as a hedge against late JSF deliveries. The service will buy 41 more F/A-18s in Fiscal 2012-14.

Meanwhile, in a major breakthrough for advocates of long-distance airpower, Gates strongly endorsed a program for “a new long-range, nuclear-capable penetrating bomber.” The Air Force has been struggling to get this program reinstated since Gates deferred development of the so-called “2018 bomber” in 2009, against the opposition of some senior Pentagon leaders who argued that smaller unmanned aircraft, plus cruise and ballistic missiles, could adequately supplement existing bombers in the foreseeable future.

Gates also announced decisions on a number of controversial aspects of the new aircraft. It will be nuclear-capable — some had argued for this, on the grounds that radiation-hardening is relatively inexpensive at the design stage and costly to retrofit, while others had opposed it because it brings the bomber within the scope of arms-control discussions. Gates also says that it would be “optionally” piloted rather than unmanned, and that it would make use of existing technologies to speed development.