Chowns said it was important to have a record, as there were few documents or actual photographs from the era.
He is also one of the few people still alive to have worked in tactical intelligence for RAF Fighter Command in the 1950s, he said.
"I was 18, and everybody I worked with was older - they've all since died.
"I'm one of 15 oral historians for RAF Fighter Command. And I'm probably the only one out of the 15 who can draw."
Chowns also has documents from his time working in tactical intelligence and a photograph of himself and colleagues in the bunker that was against the rules to take.
He admits to taking a minor liberty with the painting, despite his painstaking research.
New Zealander Keith Park, who was in the British RAF, is depicted with "New Zealand" on his shoulder, which he would not have had.
"That's for Kiwis because I know they'll like it, and everybody will look at it and know he's a Kiwi."
Chowns said what was shown in the painting was a fictitious moment as it could not be proven whether Churchill was in the bunker at that particular time on that day.
He painted the officers as "a little bit smug" as they had just destroyed twice as many aircraft as the Germans.
It is also the moment they had just killed 3000 men, showing the sacrifice in stopping fascism, Chowns said.
"It's an awful thing to think about. It's terrible and it worries me today. I'm very concerned today with what's going on in the world."
Chowns' slashing of his own paintings at a previous exhibition was also about sacrifice.
"That was the reason, because I'm tired of the sacrifice of young people, and that was a re-enactment of a Druidic sacrifice."
He has had an email from the museum's curator in the UK, who said the painting was "magnificent" after seeing a photograph.
The painting will be on display at Reyburn House Art Gallery, and Chowns will be available to talk with the public about it, at 2pm on Thursday.