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Sources

Where the facts come from

Every factual claim on this site links to its source. This is the full list.

How we checked

Each fact was written alongside its source. Our build script stores the source text and, for every fact, checks that the key words and figures really appear in it. 163 of 163 checks passed when this site was last built (checked 29 Oct 2026). Where we couldn’t read a source, we left the claim out. Britannica is used for its main article text only.

Figures that change

Things like which aircraft are flying, fleet sizes and museum displays change. Facts like these are marked “checked 29 Oct 2026”. Historical dates and wartime figures don’t change, but sources sometimes report them differently, so we show each one.

Royal Air Force (8)

RAF Museum (1)

Imperial War Museums (5)

Other museums (4)

Britannica (2)

When sources disagree

Where reputable sources give different figures, we show all of them, with links, and say how we handle it.

How many were built?

How we handle it: Six sources, six figures. Ingenium’s 21,554 includes some 1,220 Seafires; the others don’t say exactly what they count. We quote each one with its source and never pick a “true” total.

When was it in production?

How we handle it: IWM says 1937 to 1947, Ingenium 1938 to 1948, and Britannica says production ceased in 1947.

When did it first fly?

How we handle it: The RAF, the RAF Museum and the Smithsonian all give 5 March 1936, and the RAF marked the 90th anniversary on 5 March 2026. Ingenium gives 6 March 1936 and Britannica says March 1935. We use 5 March 1936.

What did Mutt Summers say?

How we handle it: Two wordings. The RAF Museum also explains what he meant: report first, change later.

How many Spitfires were there in September 1939?

How we handle it: 306 in service (RAF Museum) or 400 (Smithsonian); ten squadrons operational (RAF) or nine fully equipped and two converting (Smithsonian).

How many Spitfire squadrons fought in the Battle of Britain?

How we handle it: 18 (IWM, by September 1940) or 19 (RAF). The two may be counting at different points in the battle; neither page says.

Spitfire or Bf 109: which was faster high up?

How we handle it: IWM says the Bf 109 was faster at high altitude; Britannica says the Spitfire was faster above 15,000 ft. Performance changed with each version of both aircraft, and these short summaries don’t say which versions they compare.

How powerful was the Mk I’s Merlin?

How we handle it: Around 990 hp as first designed and over 1,100 hp in fighter trim (IWM), or 1,030 hp (Britannica).

Mk Vc figures

How we handle it: Different engines (Merlin 45 vs Merlin 45M), different spans (Shuttleworth’s aircraft has clipped wings) and different weights. Neither page says whether its weight is empty or loaded.

How many Spitfires does the BBMF fly?

How we handle it: The BBMF’s own page and its March 2026 news say five; an older RAF Battle of Britain page says six. We use five.