U.S. Air Force to Rebuild GBU-57 Bunker Busters After B-2 Strikes on Iran Nuclear Sites.

The U.S. Air Force awarded Boeing a $61.55 million contract to replenish GBU-57 Massive Ordnance Penetrator bunker-buster weapons following their operational use against Iranian nuclear facilities. The move restores a low-density but strategically critical capability to defeat hardened underground targets central to U.S. conventional deterrence.

The U.S. Air Force has awarded Boeing a $61.55 million contract to replenish the GBU-57 Massive Ordnance Penetrator (MOP) weapon system, restoring America’s most credible conventional option for defeating hard and deeply buried targets that underpin adversary WMD programs and hardened command infrastructure. The not-to-exceed, undefinitized award covers key hardware that turns the 30,000-pound penetrator into an employable, field-ready weapon, including KMU-612 E/B tailkits, fuze-system components, separation hardware, and dedicated containers and shipping sets, with work in St. Louis and completion projected between September 2028 and May 2030.
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The U.S. Air Force has awarded Boeing a $62.55 million contract to replenish GBU-57 Massive Ordnance Penetrator bunker-buster bombs, restoring America's unique ability to destroy deeply buried and hardened targets from B-2 stealth bombers and reinforcing conventional strategic deterrence after recent operational use (Picture source: U.S. Air Force).

The procurement can be read as a reconstitution move after the munition’s first operational employment during Operation Midnight Hammer, when B-2 Spirit stealth bombers dropped 14 MOPs against Iran’s deeply buried nuclear facilities at Fordow and Natanz. Air Force documents cited in U.S. reporting describe the replenishment as “critically needed” to restore readiness after expended inventory, underscoring that this is a low-density, high-consequence capability tied directly to strategic contingency plans.

The GBU-57 is a precision-guided penetrator system optimized for hard and deeply buried targets such as bunkers and tunnels. The Pentagon’s Director, Operational Test and Evaluation describes it as a large, GPS-guided penetrating weapon and highlights the Large Penetrator Smart Fuze (LPSF) effort as a critical upgrade because it increases the probability of kill when target intelligence is incomplete or uncertain. In practical terms, the fuze and weaponeering logic matter almost as much as sheer mass, because the aim is not surface cratering but controlled, delayed detonation after the weapon has driven to the intended depth and structural layer.

The MOP’s physics are simple and brutal: survivable casing, high impact energy, and a large internal charge designed to function in confined, buried spaces. Open-source technical imagery analysis and markings indicate a high-explosive fill on the order of 5,342 pounds, including approximately 4,590 pounds of AFX-757 and about 752 pounds of PBXN-114, a mix that supports powerful internal blast and overpressure once the penetrator stops and detonates. Estimates indicate the bomb is intended to penetrate up to roughly 18.3 meters of concrete or about 60.96 meters of earth before exploding, a range that captures why the weapon remains uniquely relevant against facilities buried under mountains rather than merely reinforced roofs.

A critical operational constraint is delivery: the B-2 Spirit is the only aircraft in the Air Force inventory programmed to employ the MOP, a point reinforced by the structure of the weapon system itself: the GBU-57 is composed of the BLU-127 warhead, the KMU-612 tail kit, specific bomb rack systems, loading adapters, and separation nuts, reflecting a tailored integration rather than a generic “hang and drop” munition. During Operation Midnight Hammer, each B-2 carried two MOPs, emphasizing both the bomber’s internal carriage advantage for survivability and the limited sortie-level magazine depth of this niche capability.

Those delivery realities shape tactics. Against Fordow, satellite analysis pointed to a deliberate “double tap” approach, with multiple penetrations aligned to drive repeated weapons into the same entry points, effectively drilling deeper with successive strikes. That kind of employment demands exquisite aimpoint generation, tight timing, and the ability to re-attack precisely under contested conditions. It also demonstrates why the Air Force treats the tailkit and separation hardware as readiness-critical: without enough guidance kits, racks, containers, and fuze components, the inventory is not operationally usable even if warheads exist.

At the operational level, Midnight Hammer highlighted that MOP employment is inseparable from a wider joint strike architecture. The strike package involved seven B-2s and more than 30 Tomahawk missiles, alongside deception tactics, fighter sweeps, and suppression fires designed to clear a path into Iranian airspace without shots fired at the bombers. This is the real tactical value proposition: MOP is the final penetrating tool in a chain that includes ISR to map underground complexes, stealth platforms to survive dense air defenses, and escort and suppression assets to manage residual threats.

The GBU-57’s importance is disproportionate to its numbers because it closes a specific deterrence gap: the conventional defeat of hard and deeply buried targets that adversaries deliberately build to ride out air campaigns and preserve regime survival options. The Air Force frames MOP’s mission as destroying adversaries’ WMD located in well-protected facilities, and it provides combatant commanders with a low-observable platform-deliverable conventional hard and deeply buried target defeat capability to achieve national security objectives. In deterrence terms, that means holding at risk the underground sanctuaries that enable nuclear breakout, protected leadership continuity, and hardened C2 nodes, without immediately escalating to nuclear employment.

The contract’s timing also signals an industrial-base and planning reality: this is a sole-source, Boeing-specific production and sustainment lane for a weapon that cannot be quickly replicated by another vendor without unacceptable delays. Air Force justification language stresses that restoring the capability is essential for Air Force Global Strike Command to support strategic contingency war plans for all combatant commands. When a capability is both high-value and low-density, even a single operational expenditure event, like the 14 weapons used in 2025, creates a readiness dip that must be corrected years ahead of potential future crises because tailkits, test assets, and specialized containers do not appear overnight.

Looking forward, replenishing MOP buys time as the Air Force pursues a successor. The service is developing a Next Generation Penetrator with Boeing supporting tail-kit and all-up-round integration, aiming to improve defeat of deeply buried targets while addressing modern constraints like survivability and degraded navigation environments. The B-21 Raider, described by the Air Force as a penetrating stealth bomber designed for high-end threats and built to employ a broad mix of stand-off and direct-attack munitions, is expected to expand the set of platforms able to carry future penetrators, reducing reliance on the small B-2 fleet for this mission set.

Rebuilding the GBU-57 inventory is therefore a deliberate reinforcement of the United States’ conventional deep-strike backbone. In an era defined by hardened facilities, dispersed nuclear programs, and air defense modernization, the ability to penetrate and destroy what adversaries believe is invulnerable remains one of the most potent tools in Washington’s deterrence arsenal.

Written by Evan Lerouvillois, Defense Analyst.

Evan studied International Relations, and quickly specialized in defense and security. He is particularly interested in the influence of the defense sector on global geopolitics, and analyzes how technological innovations in defense, arms export contracts, and military strategies influence the international geopolitical scene.