Langham Dome in north Norfolk, one of only six remaining training domes in the country, was built in 1942 and sits on the edge of a former RAF base.
Film of enemy planes was projected onto its walls for target practise.
A museum in the restored concrete structure is now open to visitors following grants from English Heritage and the Heritage Lottery Fund.
Listed as a Scheduled Ancient Monument in 1986, plans to restore the dome at the former RAF Langham, seen at the forefront of both defence and attack during WW2, began more than 20 years go.
The restoration of the 25ft (7.6m) tall and 40ft (12m) wide dome got under way in 2013 after the project secured grants of nearly Â£650,000.
One of the key issues was to protect the metal grid framework on which the dome is based, said the Friends of Langham Dome.
Extensive work, involving 17 tonnes of rendering sand on the shell, was also required to ensure it was watertight.
Malcolm Crowder, from the North Norfolk Historic Buildings Trust, said: "The dome was built essentially as a temporary structure over 70 years ago.
"Crumbling concrete on the outer surface had allowed water to get in, causing serious deterioration to the metal framework. This became a vicious circle of decay for both the concrete and metal structure.
"This situation has been reversed and the building... is now fit to grace the Norfolk skyline and educate the public for at least another 50 years. It's a truly outstanding heritage project achievement."
The Langham Dome Museum is open until the end of October.

6 January 2017 Last updated at 07:14 GMT
Louis Nethercott, 27, from Wiltshire, was medically discharged from 42 Commando in Bickleigh, Devon, last year after struggling to cope with seeing his colleagues killed in Afghanistan.
Alongside another former Marine, Anthony Lambert, the pair are back in the UK after completing a gruelling trek across Borneo.
Louis, who grew up in Bristol, has four more islands to go - Papua New Guinea, Madagascar, Greenland and Baffin.

East Sussex Fire and Rescue Service said it was called at 18:39 BST and two fire engines were sent to the scene to assist.
The i360 pod was carrying a private party, including a heavily pregnant woman, who boarded the pod at about 17:00.
It was lowered to the ground about two hours later.
The BAi360 Twitter account said: "Our engineers are resolving the problem & expect it to be working normally soon."
A local firm of building and quantity surveyors, MacConvilles, tweeted saying: "Our Director is enjoying the hospitality of the #i360. Technical hitch has halted ride but help is on its way!"
Tim Jones, one of the party organisers, said: "We weren't very high up when we juddered to a halt and we were told by staff to move to one side of the pod to 'recalibrate the weight' - which made some people a bit nervous.
"It was an interesting experience, and quite ironic as the party was organised by four local businesses who wanted to come together to celebrate the tower's opening."
"Only two interactions were made over the two hours and they kept pretty quiet about the free bar and the emergency toilet until we were nearly on the ground," he added.
Restoration on an at-risk World War Two building which was used to train anti-aircraft gunners has been completed.
[NEXT_CONCEPT]
A former Royal Marine who's recovering from post-traumatic stress disorder has completed the first stage of a challenge to cross the world's five largest islands unaided.
[NEXT_CONCEPT]
A "technical hitch" has been blamed for leaving 180 passengers stuck on British Airways's i360 attraction in Brighton.