With the Queen at the official viewing in the castle’s oak room was Air Commodore Scott Miller and other members of the RAF.
The painting shows the head of state sat at a desk in the white drawing room at Windsor Castle.
In the image she is wearing a brooch presented to her by the RAF Regiment which shows the Astral Crown and crossed Lee Enfield rifles.
After the viewing Mr Brown said: “This was the professional and personal opportunity of a lifetime. I had an overwhelming sense of being in the presence of living history.”
The painting will be displayed at RAF Honington in Suffolk, home to the RAF Police and the RAF Regiment.
The Queen recently unveiled another portrait of herself with an RAF association, a painting by award-winning artist Ben Sullivan commissioned by the RAF Club in London to mark its centenary.
Many royal fans noticed that in the background of the photographs was a never-before-seen picture of Queen Elizabeth with her grandchildren.
Most notably, Princess Charlotte sits atop Queen Elizabeth's lap.
The Queen's relationship with her great-grandchildren has been well documented, with the young royals having an adorable nickname for the 92-year-old monarch.
It was revealed the royal kids call Her Majesty 'Gan Gan.'