The SDB was conceived during the 1990s  to provide an internally carried  weapon which would allow the F-22A Raptor, and later JSF, to attack multiple targets. The design is sized so that the F-22 can carry eight rounds in its main weapon bays.

The SDB was operationally deployed last year, and first dropped in combat during a close air support sortie in Iraq, on October 5th, 2006.

Development of the SDB followed the Miniature Munitions Demonstration Technology (MMTD) program during the 1990s.

The US Air Force intends to procure 24,000 or more rounds for carriage on new and legacy combat aircraft; of these half will be baseline weapons equipped for attacking fixed targets, and the remainder a variant equipped to attack moving targets.

Design aims for the SDB included a capability to penetrate hardened targets and provide better accuracy than the JDAM. The MMTD effort included a focus on key technologies including the Hard Target Smart Fuze, High Energy Explosives, Penetrator Design, Optimal Guidance, Robust Autopilot, Differential GPS/INS, and an Advanced Seeker. The Optimal Guidance is a unique feature, it is designed to align the bomb body exactly with the weapon's velocity vector at the point of impact, as this maximises  penetration of the target - all of the bomb's kinetic energy is used to drive the weapon in - older guidance systems did not achieve this and velocity components tangential to the impact would at best waste energy, at worst contribute to premature casing rupture.

The production SDB is equipped with Rockwell Collins GPS receiver,  with a Harris anti-jam module, and a Honeywell inertial unit, a KDI Precision Products reprogrammable electronic fuse (airburst, contact and delay modes), HR Textron tailfin actuators, MBDA diamondback foldout wings, with a TAM Garland 50 lb forged casing warhead. A Mil-Std-1760 interface is used. The launcher is a Sargent Fletcher pneumatic ejector system in the Boeing BRU-61/A bomb rack.

SRI are providing differential GPS ground stations, required to enhance SDB accuracy over JDAM and other conventional munitions.

The glide wings provide a quoted delivery range of around 60 nautical miles for a high altitude release. The weapon performs a 180 degree roll post launch as the stowed configuration has the folded wings beneath the weapon.

The cited blast radius is 26 ft (cf 82 ft with 2,000-lb JDAM). Boeing claim the ability to penetrate more than 5 ft of steel reinforced concrete making the SDB competitive against the BLU-109/B for many targets.

The SDB will be most effective in the urban and broader close air support, battlefield interdiction, Destruction of Enemy Air Defences (DEAD) lethal suppression and counter-air strike airfield attack roles. Against soft skinned vehicles and structures, armour, point emplacements, runways, aircraft shelters and SAM/SPAAG systems this weapon will be highly lethal.

Where the SDB will be less than effective is against deep / hardened bunkers, large infrastructure targets, large buildings, industrial plant, bridges, large trench systems, vehicle parks, infantry on the move and other area or large point targets. These remain the domain of larger specialised bunker busting weapons, or large explosive bombs such as the Mk.83/BLU-110 (1,000 lb), Mk.84/BLU-117/BLU-119 (2,000 lb),  BLU-109/116/118 (2,000 lb), BLU-113/122 (5,000 lb).

The GBU-40/B / GBU-42/B Small Diameter Bomb II is in development, it will be equipped with a multimode terminal seeker and two way datalink, and is expected to enter production at the end of the decade.

Above Boeing GBU-39/B Small Diameter Bomb; below weapon in flight  (USAF Photo).

Specifications - GBU-39/B Small Diameter Bomb