Israel has received the first of the thousand new U.S. 250 pound (actually 285/129 kg) Small Diameter Bomb (SDB, or GBU-39) it ordered earlier this year. Apparently these weapons arrived quickly, and it's believed that some SDBs have already been used during Israeli air raids on Gaza. Earlier, the Israeli had received a few SDBs for evaluation, and some of these were apparently dropped as well.
The SDB is not cheap, Israel paid about $77,000 for each one. It was believed that Israel wanted this weapon not just for its precision, and low explosive (17 kg/38 pounds) power, but also its ability to penetrate 2.4 meters (8 feet) of concrete. This would make it useful to take out Hezbollah bunkers and rocket storage sites in southern Lebanon, and similar facilities Hezbollah helped Hamas build in Gaza.
Two years ago, the U.S. Air Force finally got the SDB into service. The SDB was supposed to enter service in 2005, in the wake of the 2004 introduction of the 500 pound (227 kg) JDAM. But there were many technical problems with the SDB. That's because this was not just another "dumb bomb" with a GPS guidance kit attached. The SDB had a more effective warhead design and guidance system. Its shape is more like that of a missile than a bomb (1.8 meter/71 inches long, 190 millimeters in diameter), with the guidance system built in. The smaller blast from the SDB results in fewer civilian casualties. Friendly troops can be closer to the target when an SDB explodes. While the 500, 1,000 and 2,000 pound bombs have a spectacular effect when they go off, they are often overkill. The troops on the ground would rather have more, smaller, GPS bombs available. This caused the 500 pound JDAM to get developed quickly and put into service.
The SDBs are carried on a special carriage, which holds four of them. The carriage is mounted on a bomber just like a single larger (500, 1,000 or 2,000) pound bomb would be. For Israel, this allows each fighter-bomber to take out up to four more targets per sortie. The latest version (SDB 2/GBU 40) has a glide range of up to 110 kilometers and lands within 8 meters (25 feet) of its aim point. The original SDB has a glide range of 40 kilometers.
The SDB is basically an unpowered missile, which can glide long distances. This makes the SDB even more compact, capable and expensive. JDAM (a guidance kit attached to a dumb bomb) only cost about $26,000. The small wings allow the SDB to glide long distances, especially from high altitude. SDB also has a hard front end that can punch through rock or concrete, and a warhead that does more damage than the usual dumb bomb (explosives in a metal casing.) The SDB is thus the next generation of smart bombs. There were 250 pound dumb bombs, but they were only useful for area bombing. It made sense to merge the guidance kit and the bomb itself to produce a weapon with more penetration and range. But the superiority of guided bombs is such that the next generation of heavier (500-2000 pound) smart bombs will probably be like the SDB.