Taranis, named after the Celtic god of thunder, was today billed by military chiefs as the most technologically advanced aircraft ever built in the UK.
The project, which has so far cost £185 million - funded jointly by the Ministry of Defence and UK industry, will be able to launch precision strikes in hostile territory while remaining undetected.
But bosses today said that although the aircraft could fly itself autonomously, it would not be used in that way - and would not be able to set its own missions.
Taranis was first unveiled in July 2010, but has remained classified until now.
Today at a briefing in London, the MoD and BAE Systems announced that the aircraft - described as a "combat vehicle demonstrator", designed to prove that the technology it is using works - completed its first flight trials last year and said it had surpassed all expectations.
Nigel Whitehead, group managing director of BAE Systems, said in a maiden 15-minute flight in August at a secret location, Taranis carried out a perfect take-off, rotation, "climb-out" and landing, piloted remotely by former RAF pilot Bob Fraser.
Mr Whitehead said a number of flights, lasting up to one hour each, and at a variety of altitudes and speeds, were carried out last year - but could not confirm exactly how many.
He said: "The aircraft has been designed to demonstrate the UK's ability to create an unmanned air system which, under the control of a human operator, is capable of undertaking sustained surveillance, marking targets, gathering intelligence, deterring adversaries and carrying out strikes in hostile territory."
Britain already uses drones, mainly for intelligence gathering, although some are armed, but Taranis - which could eventually be built and used in the 2030s - would be the first specifically-designed unmanned combat aircraft designed to fly in contested airspace.
Philip Dunne, minister for defence equipment, support and technology, said: "This is the most advanced air system yet conceived, designed, and built in the UK and it is vitally important for the future both of UK air defence and the UK defence industry."
"Taranis is providing vital insights that will help shape future capabilities for our armed forces in coming decades. Its advanced technology is testament to the UK's world leading engineering skills that keep Britain at the cutting edge of defence."
Air Vice Marshal Sue Gray, who is director of Combat Air at Defence Equipment and Support, said: "We set out to build a world-class stealth unmanned air system, or remotely-piloted air system - lest we forget there is a man or a woman in the loop."
And she said that had been successful, with Taranis representing an "important milestone on a road map for the future".
She added: "We are in a much stronger position as we begin the forward journey.
"Certainly from where we started in 2006, we have moved on in leaps and bounds.
"We are unequivocally on the right track."
Ground testing of the Taranis demonstrator began in 2010 at BAE Systems' military aircraft factory in Warton, Lancashire, and in April last year taxi trials were carried out on the runway.
The aircraft and its ground station were then shipped to a test-range in a so-far undisclosed location outside the UK, before the first flight took place in August.
Unmanned aircraft, or "drones", remain a controversial subject - with fears that technological advances would see aircraft flying themselves and choosing their own targets.
But Air vice marshal Gray said that would not be the case and, although Taranis could technically fly itself autonomously, it would not be operated in that way.
She said every test flight had been carried out with a pilot controlling it.
"We are talking about there being a man in the loop. The aircraft is capable of flying itself, that's not the way it would be operated.
"It is capable technologically of flying autonomously but that's not the way it would be operated.
"The technology is there for it to fly an automated route but that is not what you do when you fly a mission. There will always be a man in the loop."
"It can only fly once it has been programmed by a person. It can't make up its own missions."
She said Taranis would be an important addition to Britain's fleet of manned and unmanned aircraft and, importantly, had a focus on "low observability".
"When you start going into contested airspace you need to have every advantage," she said.
She said Taranis had not been tested with a full weapons system yet, but would probably have a "full range" of weapons.