Within months of the Cuban Missile Crisis, weapons designers at Los Alamos National Laboratory began engineering what would become the longest-serving and most adaptable weapon in America’s nuclear arsenal. Although it was ballistic missiles that had sparked the crisis in Cuba and came to symbolize the nuclear age, the new B61 would be an air-delivered gravity bomb. So named because it is dropped from an aircraft, gravity bombs had been a feature of aerial warfare since wooden biplanes released them over the trenches in World War I. But the B61 was a revolutionary weapon, featuring a versatility not seen in a nuclear munition before or since.
Sixty years later, it remains the primary family of gravity bombs in the U.S. nuclear stockpile. The B61’s longevity attests to both the timelessness of its role and the modularity of its original design. Indeed, there is no longer a single B61 today but an ensemble of weapon systems that all derive from the same progenitor. Thirteen variants, or modifications, have been developed, each with distinct characteristics and missions. A variety of modifications can be delivered by a broad range of U.S. and allied aircraft, from strategic platforms like the B-2 stealth bomber to dual-capable fighters such as the F-15, F-16, F-35, and the European Tornado, as well as next-generation systems like the B-21 Raider. In various configurations, the bombs can be released in free-fall mode or with a speed-retarding parachute that when packed is denser than hardwood.
The latest descendant in this bloodline is the B61-13, whose first production unit the National Nuclear Security Administration heralded this week as part of the comprehensive modernization of the U.S. nuclear stockpile. As stewards of the stockpile, my colleagues and I are responsible for ensuring the unerring reliability of these systems, as well as reconfiguring them when necessary to meet evolving threats. Although nuclear deterrence has shown remarkable consistency through the decades — the triad of land-, sea-, and air-based weapons is more or less the same today as it was in 1960 — nuclear weapons are periodically tailored in response to fluctuations in the security environment. Just as adversaries adjust how they protect the assets they value most, the United States calibrates its tools to hold them perpetually at risk. The B61-13 is the most recent manifestation of this principle. Designed to provide enhanced capabilities against certain harder and large-area military targets, or in Pentagon parlance to “deny an adversary sanctuary from attack,” the B61-13 is a steppingstone toward a more definitive solution to a longstanding military challenge.
Defeating hardened targets is a perennial aspiration in modern warfare. During World War II, the German Kriegsmarine constructed a naval base at Lorient, on the Atlantic coast of France, that was all but impervious to Allied bombing. Built with 3.4 million cubic feet of concrete — compared to the Hoover Dam’s 4.4 million — and featuring a seven-layer, 25-foot reinforced roof, the base’s submarine pens enabled the predations of German U-boats across the Atlantic. In hundreds of sorties, Allied bombers pummeled the fortress mercilessly from the air, including with 12,000-pound “Tallboy” ballistic bombs, to no avail. In exasperation, the Allies finally shifted to pulverizing the surrounding city to kill essential personnel and cut supply lines to the base, which remained insuperable until the very end of the war.
In the following decades, militaries the world over built command centers and other facilities that were not only heavily reinforced but buried deep beneath the Earth, presenting a vexing problem to targeteers. Naturally, strategists looked to nuclear weapons as a potential solution. If a system could be designed to penetrate soil, rock, and concrete before detonating, the effects would greatly increase the efficacy of the weapon against the most forbidding structures. As early as 1952, the Mark 8 nuclear bomb was introduced to the U.S. stockpile with the means to reach and destroy such targets.
Yet, as weapons capabilities advanced, so did feats of structural engineering to protect against them. The Moscow Metro, famously deep by the standards of the world’s subways, was designed in part to double as a massive bomb shelter for the Kremlin leadership, motivating ever more powerful weapons in the U.S. stockpile to compromise it. Today, the challenge is even more daunting. Weapons designers are responding to dictates from the highest levels of the U.S. government to remove the possibility that enemy assets can be placed beyond the reach of America’s nuclear force.
The last two Nuclear Posture Reviews — periodic presidential directives on the role of nuclear weapons in U.S. national security — have emphasized the defeat of especially stubborn targets. President Donald Trump’s first Nuclear Posture Review, issued in 2018, cited North Korea’s reliance on hardened and deeply buried facilities to secure the Kim regime, pledging to “field a range of conventional and nuclear capabilities able to hold such targets at risk.” President Joe Biden’s, released in 2022, echoed the theme, vowing to “leverage existing capabilities to hold at risk hard and deeply buried targets” and “develop an enduring capability for improved defeat of such targets.” Congress, too, has joined the chorus. The Fiscal Year 2023 National Defense Authorization Act directed a study on options to destroy hard and deeply buried targets, including a “proposed strategy for fielding such capabilities in sufficient quantities and making other adjustments to the strategy and plans of the United States to account for the growing hard and deeply buried target set…”
The B61-13 represents an intermediate answer to these edicts as even more effective instruments are being evaluated. While the bomb will provide new military capabilities, it will not be a novel weapon in the sense of being assembled from wholly new components and materials. Rather, the system is a modification, a subset of weapon modernizations that upgrade existing warheads with new operational characteristics. Most of the weapons in the U.S. nuclear arsenal were built in the 1970s and 1980s, and the B61 is one of seven systems undergoing modernization to extend their operability or add further capabilities. The planned W93 warhead, by contrast, will be an altogether new, modern weapon.
As with any complex electro-mechanical system, components and materials in nuclear warheads degrade over time. To ensure their continued safety and reliability, engineers scrupulously analyze every component — more than 6,000 individual parts in the case of the B61 — and determine whether to reuse, repair, or replace them. The modernization process resolves aging and performance issues, enhances safety features, improves the security of the weapons, and can, as in the case of the B61-13, impart new military attributes.
Among the most remarkable aspects of the B61-13 program is the speed with which it is being accomplished without disruption to the broader modernization campaign. The first usable bomb was completed almost a year before the original target date and less than two years after the program was announced. For a nuclear enterprise often seen as operating at a languid pace over the last two decades, this was the latest achievement that signaled a newfound aggressiveness by the National Nuclear Security Administration. In 2023, the agency delivered more than 200 modernized weapons to the Navy and Air Force, the most in a single year since the end of the Cold War. Mindful of the deteriorating security environment, program managers made a number of bold decisions, often consciously accepting calculated risks, to accelerate the manufacture of the B61-13.
To be fair, compression of the timeline was enabled in part by the earlier B61-12 life extension program, which completed its final unit in late 2024. By taking advantage of production equipment and processes from the earlier program and using common components, the B61-13 progressed much more quickly than planned. The program largely sidestepped the steep learning curve that attends a new modification. Normally, technical teams must be built and trained to perform development, engineering, testing, and production, but by harnessing the same engineering cadre and methods from the previous program, the design timeline was greatly reduced. Yet, these factors do not paint the full picture. The outcome hinged to a large degree on adjustments to the risk tolerance of the scientists and engineers managing the program, who are, by nature and training, a technically conservative bunch.
Although they maintained their customary vigilance against safety and security risks, the engineers were more forgiving of programmatic risk in the interest of moving swiftly. By streamlining, and in some cases combining, “design gates” — the exacting reviews performed at each stage of a weapon design process — the engineers significantly shortened the timeline to begin production. These economies resulted in B61-13 hardware “test builds” beginning merely three months after the program received authorization and appropriation from Congress. Further efficiencies were then devised for the production process itself.
In a typical modernization program, before the first unit of a new weapon is ever produced, it must pass an exhaustive series of qualification tests to ensure the system will function reliably in every environment. These assessments involve a combination of computer modeling and simulation as well as physical tests of weapon components. The marquee step in the process is to conduct “flight tests” in which high-fidelity replicas of the bomb — with all the internal gadgetry but none of the fissile material — are dropped from the aircraft that delivers the real thing. For the B61-13, these qualification tests are occurring in parallel with the production of live units. To the uninitiated, it is impossible to appreciate the unorthodoxy of this approach, or the willpower required for the engineers to relax their usual meticulousness. Reassured by decades of B61 design and qualification data, program managers developed a phased plan for simultaneous qualification and manufacture that allowed the standard schedule to be cut in half. While there was some risk involved, the earlier program inspired confidence that it was minimal, and well worth the accelerated delivery.
Vindication of these practices may allow them to be transposed to future modernization efforts, with implications for their production timelines. So, too, might other aspects of the B61-13 program. In particular, the process featured an extraordinary level of cohesiveness between the laboratories that designed the weapon and the plants responsible for its production. In earlier eras, relations between elements of the nuclear complex were often marked by institutional rivalries and cultural idiosyncrasies that subtracted from efficiency. The B61-13 is very much a team effort. The Los Alamos and Sandia National Laboratories in New Mexico manage the physics and systems engineering, while production is performed at the Pantex Plant in Texas, Kansas City National Security Campus in Missouri, Y-12 National Security Complex in Tennessee, and Savannah River Site in South Carolina. Experts from each site collaborated closely from the beginning, consciously avoiding the stove-piping that has characterized weapons programs in prior decades.
When complete, in addition to having its service life extended, the B61-13 will boast a greater accuracy than its forebearers by virtue of a modern “tail kit assembly” — the fins that guide and stabilize the weapon during its descent. The bomb’s yield will be higher than its older sibling, the B61-12, contributing to its utility against certain harder and large-area targets. Owing partly to these advantages, the number of B61-12s built was lowered by the same number of B61-13s that will be produced. Consequently, the new modification will not increase the overall number of weapons in the U.S. stockpile.
Of course, at some point, America’s leaders may deem it necessary to build additional nuclear weapons to meet military requirements. Russia, China, and North Korea are aggressively expanding and modernizing their nuclear capabilities with lavishly funded construction projects and weapons programs. As the United States confronts the challenge of deterring multiple nuclear-armed states simultaneously, its leaders will continuously appraise the suitability of the U.S. nuclear posture to this task. But what is abundantly clear, even without an uptick in the size of the stockpile, is that qualitatively new capabilities will be necessary to hold adversary targets at risk.
America’s nuclear modernization program is already set to deliver a force with greatly enhanced diversity of weapon characteristics and delivery vehicles. The planned nuclear-armed sea-launched cruise missile, for instance, will provide the Navy with considerable new optionality to deter China in the Indo-Pacific. Work is well underway on the Air Force’s Long-Range Standoff cruise missile. Should additional nuclear capabilities be needed to overcome integrated air and missile defenses or provide more proportional theater response options, warhead design and production can now be performed on a timeline unimaginable a decade ago.
Even then, there was reason for optimism that nuclear weapons would continue to recede in relevance, as they had in the decade prior. Yet the behavior of America’s adversaries in the intervening years has made clear that nuclear deterrence will remain a dominant feature of the global landscape for the foreseeable future. Deterrence is not magically achieved by reaching an arbitrary threshold of missiles and bombs — it is a condition painstakingly attained and preserved by fielding the right number of the right weapons in the right places. The B61-13 is merely the latest output of the formula used to shape America’s nuclear arsenal, whose wisdom will ultimately be affirmed if these weapons never have to be used.
David A. Hoagland serves as the National Nuclear Security Administration’s acting deputy administrator for defense programs, with responsibility for maintaining the safety, security, and reliability of the U.S. nuclear weapons stockpile. Prior to his current role, he served in other elements of the National Nuclear Security Administration and on the National Security Council staff, where he focused on countering nuclear terrorism and nuclear proliferation.
Image: Sandia National Laboratories
