An F-35A traveling at supersonic speeds successfully dropped a mock B61-12 nuclear gravity bomb from an internal compartment over the Tonopah Test Range in Nevada, the National Nuclear Security Administration (NNSA) said Monday.

The test took place in August in an F-35A Lightning II, according to a press release from the semi autonomous Department of Energy agency.

Block 4 upgrades to Lockheed Martin’s [LMT] F-35A will eventually allow the Joint Strike Fighter to internally carry a pair of B61-12 nuclear gravity bombs provided by the NNSA, and which will use guided tail kits developed by Boeing [BA] to give the weapon what the Air Force has called “modest standoff capability.”

It will take nearly until the end of the Block 4 program, which last year slipped two years into 2026, to certify F35-A as a dual capable aircraft: one that can carry conventional and nuclear munitions.

B61-12 will homogenize four existing versions of the bomb, which is the oldest deployed weapon in the U.S. arsenal. The NNSA plans to build some 480 B61-12 bombs, the nongovernmental Federation of American Scientists in Washington estimates.

Including Air Force and NNSA work, the B61-12 will cost between roughly $11.5 billion and $13 billion over about 20 years, according to documents from DoE and the Department of Defense. The NNSA’s share of the bomb’s cost is about $8 billion, the agency estimates.

NNSA has said it will produce its first war-usable B61-12, the first production unit, in November 2021. The first production unit milestone slipped more than a year after the NNSA acknowledged in 2019 that electrical capacitors needed by several bomb components had to be replaced.