Although the 250 pound Small Diameter Bomb (SDB) has not yet gone into service, the U.S. Air Force is already working on a new version. The SDB was supposed to enter service in 2005, in the wake of the 2004 introduction of the 500 pound JDAM.
The SDB is a 250 pound bomb with a more effective warhead design and guidance system. It's shape is more like that of a missile than a bomb (70 inches long, 190 millimeters in diameter). The smaller "bang" will result in fewer civilian casualties when an SDB is used in an urban area. 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. But what the air force really wants is to equip the B-1 with SDBs, as this bomber could carry as many as 216 of them. The new F-22 and F-35 warplanes are stealthy and normally carry their bombs internally. This limits how many they can carry, but with the SDB, an F-22 can carry eight of them. The Navy F-18 could easily carry 24 SDBs.
The SDB is also unique in that it does not look like the larger JDAM series of smart bombs, which are conventional 500, 1,000 and 2,000 pound bombs with a smart bomb kit attached. SDB is more like an unpowered missile, with the guidance and control equipment built into the bomb. This makes the SDB even more compact, capable and expensive (about $70,000 each.) JDAM (a guidance kit attached to a dumb bomb) only cost about $20,000. The small wings also allow the SDB to glide up to 70-80 kilometers (from high altitude.) SDB also has a hard front end that can punch through several feet of 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 was never any no point in building a 250 pound dumb bomb, as they would be too inaccurate to be useful. So it made sense to merge the guidance kit and the bomb itself. 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.
The problem is that the SDB has required more testing that expected, and it's availability for combat has been delayed several times. While the SDB only costs $30,000 to manufacture, development costs are more than doubling the price. The next version of the SDB will have an additional seeker on the front of the bomb to provide even more accuracy (a meter or so, as in sending the bomb through a window or door.) The new version of the SDB (SDB 2), which can track and hit moving targets, is expected to cost about $90,000 each. That's not counting development costs, which could be substantial. The SDB 2 is not expected to enter service for another eight years. The original SDB has been in development for five years so far.