Air Force contractor Boeing [BA] is clear to start production on crucial hardware for the B61-12 next-generation nuclear gravity bomb, which is scheduled to be built in 2020, the service said recently.

Boeing’s tail-kit cleared Milestone C — which in Pentagon program management precedes a project’s construction phase — in late October, the Air Force Nuclear Weapons Center at the Kirtland Air Force Base in New Mexico wrote in a press release Friday. The service has already tested the design in 2018 and 2017 using dummy bombs carried by the B-2 bomber and the F-15.

That clears Boeing to start building the tail kit in 2020, the same year the Department of Energy’s semi-autonomous National Nuclear Security Administration (NNSA) expects to produce its first war-ready bomb. NNSA started work on the first production B61-12 in October.

While NNSA will build the bomb’s assembly and explosive package, the Air Force must build B61-12’s guided tail-kit assembly, which helps control the weapon during free fall. The Air Force is also responsible for integrating the NNSA-made bomb with carrier aircraft, including versions of the B-2, the planned B-21, F15, F-16, F35 and the German-made PA-200, according to the Air Force Nuclear Weapons Center.

B61-12 will be an adjustable-yield gravity bomb that replaces four previous iterations of the storied old weapon, which was designed by the Los Alamos National Laboratory and first built in 1968. Including Air Force and NNSA work, the weapon with its tail kit will cost between roughly $11.5 billion and $13 billion over about 20 years, according to NNSA and Pentagon documents.

NATO members use the relatively low-yield B61 bomb as a regional deterrent against Russia.

The Air Force expects the guided tail-kit assembly to cost a little under $3 billion to complete over roughly 20 years, according to a 2016 Pentagon audit. Boeing got its Air Force contract to build the tail kit in 2012.

Los Alamos and Sandia are the lead labs on NNSA’s B61-12 life extension program. NNSA formally started work on the B61-12 life-extension program in 2015 and plans to begin a five-year production run in 2020. The civilian agency thinks its share of the life extension program will cost a little less than $8.5 billion, according to its latest Stockpile Stewardship and Management Plan. The non-government Federation of American Scientists estimates NNSA will produce 480 bombs.

B61-12 will not need new plutonium pits — primary warhead cores — from Los Alamos, but it will need new uranium secondary stages from the agency’s Y-12 National Security Complex in Oak Ridge, Tenn., and new non-nuclear components from the Kansas City National Security Campus in Missouri. NNSA’s Pantex Plant in Amarillo, Texas, will assemble the weapons.

The B61 is a politically charged weapon.

During the Obama administration, Democrats including nuke hawk Sen. Dianne Feinstein (D-Calif.) voted for a 30-year nuclear modernization plan that included the roughly 20-year B61-12 life-extension, with the understanding that the modernization would trim the overall U.S. warhead count and eliminate the megaton-class B83 gravity bomb that the Trump administration in February decided to preserve.

Congressional Democrats have also said the soon-to-be-available B61-12 eliminates the need for the low-yield, submarine-launched ballistic-missile warhead the Trump administration called for, and for which Congress provided $65 million in development funding for the 2019 fiscal year that began Oct. 1.

The White House says the low-yield submarine warhead has a longer range than the B61-12’s carrier aircraft, and can strike targets more quickly after launch. The administration argues the U.S. must create such a weapon to stop Russia from using similar nuclear weapons Moscow already has.