The second X-37B returned to earth on June 16th, after over 15 months (469 days) in space. This feat was made possible by the fact that this X-37B carried with it a large solar panel, which came out of the cargo bay, unfolded, and produced enough power to keep the X-37B up there twice as long as the first one. The air force has not said what the X-37B was doing up there all this time. The air force has revealed that it is designing an X-37C, which would be nearly twice the size of the X-37B and able to carry up to six passengers. Think of it as Space Shuttle Lite but robotic and run by the military, not NASA. This has the Chinese worried, and they are not being quiet about their fears.
Little is publicly known about what either X-37B was doing up there. The best guess is that they were testing the endurance of new satellite components and the X-37B itself. That does not give amateur astronomers much to look at. The international collection of amateur sky watchers have proved remarkably adept at spotting orbital objects in the past, including classified ones like the X-37B, but not this time. The air force said this flight was simply to test the aircraft but would not say what, if anything, was in the cargo bay for the first flight. It was revealed that the second one took up a folded solar panel. No details on what other items were tested. The amateur orbital observer community has concluded that one thing the X-37B tested was how well it could constantly switch positions and remain hidden. In that respect the X-37B was a resounding success. That's because these amateur observers are generally very good at tracking what's up there.
The X-37B is a remotely controlled mini-Space Shuttle. The two X-37Bs, according to amateur astronomers (who like to watch spy satellites as well) appeared to be going through some tests. The X-37B is believed to have a payload of about 227-300 kg (500-660 pounds). The payload bay is 2.1x1.4 meters (7x4 feet). As it returns to earth it lands by itself (after being ordered to use a specific landing area). The X-37B weighs five tons, is nine meters (29 feet) long, and has a wingspan of 4 meters (14 feet). The Space Shuttle is 56 meters long, weighs 2,000 tons, and has a payload of 24 tons.
The X-37B is a classified project, so not many additional details are available. It's been in development for twelve years but work slowed down for a while because of a lack of money. What makes the X-37B so useful is that it is very maneuverable, contains some internal sensors (as well as communications gear), and can carry mini-satellites or additional sensors in the payload bay. Using a remotely controlled arm, the X-37B could refuel or repair other satellites. There is little confirmed information about its payload or mission (other than testing the system on its first mission).
The main mission of the X-37B is apparently to serve as a scale model for testing of the larger X-37C design. But there's apparently been no actual construction of the first X-37C yet, while the air force seems to have plenty of work for the two X-37Bs that already exist. The X-37C is supposed to be nearly twice the size of the B model and able to carry six people or a ton of cargo. The larger X-37C could also do intelligence work and perhaps service existing spy satellites (which use up their fuel to change their orbits). The X-37C would probably be capable of serving as a platform for attacks on enemy satellites in wartime.
There are some other similar UOVs in the works as well.