The Pentagon unveiled yesterday its fiscal year 2013 budget proposal–which includes a restructuring of the F-35 Joint Strike Fighter program–giving lawmakers details on proposed weapons changes they will challenge starting today.

The Senate Armed Services Committee (SASC) will kick off the fiscal year 2013 defense budget hearings with Pentagon leaders today, when the administration’s proposal to restructure Lockheed Martin’s [LMT] F-35 program will be scrutinized. The Pentagon wants to save $15.1 billion by delaying plans for buying 179 F-35s over the next five years, with 13 of those aircraft cuts coming in FY ’13, according to the defense budget proposal President Barack Obama sent Congress yesterday.

The Pentagon is “fully supportive” of the F-35, “but it is too concurrent (with overlapping aircraft testing and purchasing) and in our view we need to give more time for testing to be completed,” Comptroller Robert Hale told reporters yesterday. The Pentagon is standing by long-term plans to buy a total of 2,443 F-35s for the Air Force, Navy, and Marine Corps.

The Pentagon’s proposal to cut $1.6 billion for the 13 F-35s from its FY ’13 plans amounts to the largest weapons program change in its $525.4 billion base budget proposal.

The Pentagon spending plan is $45 billion less than previously projected, thanks to a deficit-reduction plan Congress approved last year. A sizable chunk of that reduction–$18 billion–comes from procurement accounts and a $6 billion drop comes from research and development spending. The budget proposal also lays out $259 billion in cuts to previous plans for spending across the Future Years Defense Plan (FYDP), which runs from FY ’13 to FY ’17. That five-year cut includes $94 billion to weapons purchases and $17 billion to research funding.

Hale defended the size of the FY ‘13 cut to procurement accounts, pointing to the withdrawal of troops from Iraq and Afghanistan.

“It’s common to see that procurement in the early parts of a drawdown is disproportionate cut, because frankly it takes a while before we can make decisions about force-structure cuts, get people out, and realize the savings from them,” he told reporters at the Pentagon. Looking out of the five-year plan to FY ’17, the procurement reduction is more proportional, he said.

Republican lawmakers, though, have been busy scrutinizing proposed cuts to weapons plans and lamenting the level of funding in proposed defense budget, which reflects spending caps set by the Budget Control Act of 2011.

House Armed Services Committee (HASC) Chairman Howard “Buck” McKeon (R-Calif.), whose committee will examine the budget during a lengthy hearing tomorrow, charge the budget fails to “adequately address threats posed by our adversaries around the world.”

“Despite the goal of pivoting to Asia, a theater where naval assets are decisive, the budget calls for retiring nine ships and removes sixteen more from the new construction plan,” he said, reflecting concerns about shipbuilding cuts expressed by other HASC Republicans.   

The Pentagon unveiled details yesterday of $75 billion in savings over the FYDP, including a $13.1 billion hit to shipbuilding. Other cuts across the five-year plan include $4.3 billion to the SSBN(X) ballistic missile submarine and $1.3 billion to the Army Ground Combat Vehicle (GCV) development efforts.

Proposed program terminations, projected to save $9.6 billion over the FYDP, include the Global Hawk Block 30 aircraft, the C-27J Joint Cargo Aircraft, Humvee recapitalization efforts, the C-130 Avionics Modernization Program, the Defense Weather Satellite System, and the Medium Range Maritime UAS. Additional program restructurings are proposed for the Family of Medium Tactical Vehicles, Joint Air-to-Ground Missile, MV-22 Osprey, P-8A Poseidon, and E-2D Advanced Hawkeye.

Capabilities the Pentagon says it will continue investing in include 11 aircraft carriers, the big-deck amphibious ships fleet, and the Air Force bomber fleet.

The new Pentagon budget begins making spending reductions in line with the Budget Control Act. The $259 billion cut across the FYDP  comprises the first part of the 10-year cut of $487 billion to previous Pentagon spending plans ushered in by the law Congress and Obama approved last August.

However, the budget the Pentagon unveiled yesterday, like Obama’s entire FY ’13 budget for the federal government, does not reflect even-larger spending reductions that may or may not result from the Budget Control Act. The law says if Congress could not craft a plan to cut $1.2 billion in additional spending last year, which it did not, so-called sequestration cuts would start next year that would add roughly another $500 billion in Pentagon cuts for a total of nearly $1 trillion over a decade.

Obama does not support the sequestration cuts and Pentagon leaders have warned they would decimate the military. The president has called on Congress to go back to work and craft a large-scale deficit reduction plan this year that would prevent them from starting next year.

With Congress at odds over how to shrink the deficit, Republicans in the House and Senate have crafted smaller-scale plans–greeted coolly by Democrats–to prevent the first year of sequestration cuts by shrinking the federal workforce.

Sens. John McCain (R-Ariz.) and Lindsey Graham (R-S.C.), who are pushing the workforce-reduction plan , said in a statement yesterday they are “deeply disappointed” Obama’s defense budget plan does not seek to avoid the sequestration cuts.

“It is past time for the president to stop leading from behind and step up, heed the words of the military and civilian defense leaders that he appointed, and work with us avoid these defense cuts that threaten America’s national security,” said McCain, the SASC ranking member, and Graham, a committee member.

The senators said the $487 billion cut to planned defense spending over the next decade, which will come regardless of the sequestration reduction, “will be painful” and they “join our colleagues in scrutinizing them carefully.”

Congressional Democrats were more muted than their GOP counterparts in reacting to the Pentagon budget unveiling yesterday.

HASC Ranking Member Adam Smith (D-Wash.) lauded the Obama administration for crafting a new defense strategy before the budget.

“I have consistently said that we can rationally evaluate our national security strategy, our defense expenditures, and the current set of missions we ask the military to undertake and come up with a strategy that enhances national security by spending taxpayer dollars more wisely and effectively,” Smith said in a statement. “I believe this budget meets that goal.”

The Aerospace Industries Association (AIA), meanwhile, called Obama’s budget proposal “a direct hit” at the “American aerospace and defense worker.”

“The budget released today takes direct aim at the first wave of 350,000 aerospace and defense workers who will be out of work if Congress does not find a solution to the sequestration trigger being pulled in 321 days,” AIA President and Chief Executive Officer Marion Blakey said yesterday in a statement. “In the meantime, hundreds of companies that together form the ‘defense industrial base’ have already begun to downsize in response to the cuts already enacted.”

In addition to the $525.4 billion base budget, the Pentagon also is seeking $88.5 billion in so-called Overseas Contingency Operations (OCO) war funding for FY ’13. That is down from $115.1 billion in FY ’12 OCO spending.

The Pentagon has just a placeholder figure of $44.2 billion in OCO funding beyond FY ’13. Hale acknowledged the projection that OCO funding will be cut in half next year “is not necessarily realistic” and will depend on level of “troops on the ground.” He said the Pentagon will make a specific proposal for the future war funding next year “that may well be different than that.”