A Thunderbirds-style military vehicle designed for unmanned, long-distance underwater missions has completed in-water testing.

The Manta Ray prototype uncrewed underwater vehicle (UUV) completed the full-scale testing off the coast of Southern California over the last three months, according to manufacturer Northrop Grumman.

Manta Ray was built through the U.S. Department of Defense's Defense Advanced Research Projects Agency (DARPA) program.

Although its future missions have not been revealed, Northrop Grumman describe the vehicle as a 'new class of UUV'.

It moves through the water using 'efficient, buoyancy-driven gliding' and could reduce need for as much human involvement in underwater missions.

Manufacturers added: 'It is an extra-large glider that will operate long-duration, long-range and payload-capable undersea missions without need for on-site human logistics.'

Testing of at-sea hydrodynamic performance included submerged operations using all the vehicle's modes of propulsion and steering - buoyancy, propellers, and control surfaces.

'Our successful, full-scale Manta Ray testing validates the vehicle's readiness to advance toward real-world operations after being rapidly assembled in the field from modular subsections,' Dr. Kyle Woerner, DARPA program manager for Manta Ray said. 

'The combination of cross-country modular transportation, in-field assembly, and subsequent deployment demonstrates a first-of-kind capability for an extra-large UUV.'

Northrop Grumman say the prototype was built in Maryland before being shipped in subsections to its test location in California.