The US Air Force is developing a super-fast weapon that can fly many times the speed of sound.

Travelling at over Mach 5.1, the jet could make it from London to New York in an hour. But it will be used instead as a super-fast weapon, working like a missile.

"We are the Air Force. What do we want to do with this technology? We want to weaponise it," Ryan Helbach, an official with the Air Force Research Laboratory, told Military.com. It’s unclear how it would be used as a weapon, but US officials expect that China and other countries are building similar technology.

The X-51 WaveRider, developed by Boeing, works by dropping out of a traditional plane. During successful tests, it flew through the air using almost no moving parts and a special air-breathing system that allows it to fly so fast.

The plane must be given a kickstart to get going, so is first propelled by an attached rocket booster. That takes it up to about Mach 4.5, and then drops off — the X-51 can do the rest of the work from there, accelerating up to Mach 5.1.

It was tested in 2013, when the plane flew at almost 4,000 mph and reached 60,000-feet before it ran out of fuel and dropped into the sea.

Boeing is now working with the Defense Advanced Research Projects Agency to shrink it into a weapon that could be attached to bomber planes.

When that happens, the plane will be able to strike targets without pilots and aircraft ever having to get close to them.