Artist's impression of Lockheed X-35A

The US needs 3,000 aircraft to take the place of its F-18s, F16s, A-10s and Harriers AV-8s.

JSF will have an empty weight of between 22,000lb and 24,000lb and a maximum take-off weight of 50,000lb.

It can carry more than 15,000lb of fuel internally, giving it a combat radius of 600 nautical miles.

As a stealth aircraft, it has internal missile bays for its 13,000lb payload, although it can carry some of that weight externally if radar detection is not an issue.

Boeing and Lockheed Martin's designs differ in their approach to achieving STOVL capability.

Stealth queried

"Boeing's design is broadly similar to a Harrier engine with three rotating nozzles," said Nick Cook, Aviation Editor with Jane's Defence Weekly.

"These point back in normal flight and point down in landing.

Lockheed's design includes a fan to aid vertical landing

"Lockheed Martin's design has a normal F-16 type engine at the rear but a separate little engine behind the cockpit.

"This is a 'lift fan' clutched to the main engine and to land vertically the pilot uses it to lower gently on a jet of air," he told BBC News Online.

The two aircraft also differ in shape and Nick Cook says questions have been raised about how the Boeing design will manage stealth capability.

"The front of the Boeing aircraft looks like a barn, it's got an enormous yawning whale-like air intake and that would be a very radar reflective piece of equipment.

"Boeing says it has methods to achieve stealth. It will be interesting to see what they are," he said.

Defence review

The JSF is planned to be in service for 2008 although new US President George W Bush has let it be known he plans to comprehensively review military spending and there are fears the $130bn project could be pushed back.

But Nick Cook says the military need JSF to be delivered on time.

"The US Marine Corps' planes are not getting any younger and something needs to replace them otherwise they will start dropping out of the sky"

The British Navy's Harriers are due for replacement in 2012.