The Milestone B decision approved late Wednesday by Frank Kendall, the undersecretary of defense for acquisition, means the program can enter the advanced development phase.

Electric Boat, the prime contractor for the Columbia-class program to build 12 new ballistic missile submarines to replace the current, aging fleet of 14 Ohio-class submarines, is about two-and-half-years into the design of the new class of submarines.

EB President Jeffrey Geiger called the achievement a "crucial step" forward for the program.

"Working closely with the Navy and the submarine industrial base, General Dynamics Electric Boat is leading many aspects of the Columbia-class development effort, including design, material procurement, construction, and operating-cost reduction, to achieve an affordable and effective program," Geiger said in an emailed statement.

He also credited relevant congressional delegations for their support in helping the program reach this point.

U.S. Rep. Joe Courtney, D-2nd District, attributed the achievement to years of work by many people, "especially" the employees at EB, which is in his district.

Milestone B is "just the beginning of the hard work ahead in designing and building this submarine within the already tight timeline we face," Courtney said in a statement released by his office.

Navy leaders have described the program as their top priority and also have said its tight timeline leaves no room for error. The Navy will start retiring the Ohio-class submarines in 2027, and the first Columbia-class submarine isn't expected to go on patrol until 2030.

Ballistic missile submarines carry the majority of the country's active nuclear arsenal and are part of larger $1 trillion effort to modernize the U.S. nuclear triad. The triad comprises manned bombers, long-range intercontinental ballistic missiles and submarine-launched ballistic missiles.

The Columbia submarines are expected to cost $8 billion each. Combine that with $13 billion for research and development, and the total acquisition cost for the program is estimated to be $128 billion. Kendall, the acquisition undersecretary, chose to spread out the design cost across all 12 submarines as opposed to tacking the cost onto the first ship in the class, as is common practice.

The latest estimate for the program does not assume any of the savings that could be achieved through the National Sea Based Deterrence Fund, according to Courtney, who championed the fund as a way pay for the costly program outside of the Navy's shipbuilding budget.

The Navy estimates that it will spend, on average, $15 billion for new ship construction in each of the next five years.

In a phone interview Wednesday, Courtney was quick to point to $773.1 million in advance procurement money for the Columbia program being deposited into the fund, marking the first time it's being utilized.

The money was included as an exception in the continuing resolution, passed at the end of last year, which funds the government through late April. Continuing resolutions, which fund the government at the previous fiscal year's level, usually don't allow money to go to new programs.

Courtney said the sea-based deterrence fund has various authorities, which he's advocated for, to lower the cost of the Columbia program, such as long-lead purchasing.

A Congressional Budget Office report estimated that the fund has the potential to save several hundred million dollars per submarine "by purchasing components and materials for several submarines at the same time."

"A disadvantage of the acquisition strategy is that if the Congress decided not to build all of the submarines for which the Navy purchased some materials, those materials might go unused," the report says.

Taxpayers for Common Sense, the self-proclaimed independent budget watchdog, has called the fund a "budget gimmick" that could lead to an increase in overall Pentagon spending.

If there are cost overruns, they could lead to a breach of the Nunn-McCurdy Act, which requires Congress to be notified whenever a major defense acquisition program exceeds certain cost thresholds.

U.S. Sen. Richard Blumenthal, D-Conn., defended the Columbia program's high price tag, saying in a phone interview Thursday that the "critical national security consequences of this program will justify the expense." He also expressed his confidence in EB employees to get the job done under the tight timeline.

Blumenthal and U.S. Sen. Chris Murphy, D-Conn., put out a joint statement on the Milestone B decision, saying it puts the Columbia program "on a clear path forward."

"It is great news for Connecticut and our national defense that work is proceeding for the detailed design of our nation's next generation of Connecticut-made ballistic missile submarines," the senators said.

The Columbia program reached Milestone A in 2011. It will next have to achieve Milestone C to enter the production and deployment phase.