. . . four other pieces of equipment that most senior officers came to regard as among the most vital to our success in Africa and Europe were the bulldozer, the jeep, the 2—-ton truck, and the C-47 airplane. Curiously, none of these is designed for combat.

— Dwight D. Eisenhower

This is a pretty tricky question and I would suggest the answer is rather subjective.

For example I could suggest it was the Spitfire, the plane that many credit as winning the Battle Of Britain. It was a very good aircraft that allowed the RAF to take the fight to the Luftwaffe during those critical months. It also was a very adaptable aircraft, it was converted to a photo reconnaissance aircraft - surely a very vital part of the war effort. The Spitfire even flew (just!) from aircraft carriers.

But then, the Hurricane was available in greater numbers during that time and whilst generally regarded as inferior to the Spitfire it did have some advantages.

If we look to bombers the Lancaster could carry a huge bomb load and were a key part of Harris's plans to bomb Germany. Then again we know that early bombing was inaccurate, the relatively "low tech"

Mosquito flew many precision bombing missions and was regarded as such a threat the Germans offered a 2:1 ratio for any confirmed kills. See correction in a post below. Nevertheless the Mosquito was a formidable aircraft.

On the Axis side whilst they did not have a heavy bomber such as the Lancaster the Stuka dive bomber was a big part of Blitzkrieg early in the war.

One could even suggest the C-47 (Dakota) that delivered paratroopers to the battle ground and transported many, many tonnes of supplies that were so vital to many campaigns.

You mention the Mustang, its role in bomber escort was important - as the US decided to bomb by day their losses were high, without the Mustang to protect them its quite possible that losses would become unsustainable, which would mean further bombing operations would not be possible. It definitely had a role to play.

Maybe the B-29 that dropped the A-bomb and made Japan cede ? Thus ending any further conflict and loss of life ?

We could go on all day and start looking at the lesser well known aircraft.

I think its difficult to pick any one aircraft that was overall "the most crucial". Each aircraft had a role to play and each one "did their bit". The availability of the aircraft was also vitally important - if push came to shove you'd rather have 100 Hurricanes at your disposal than say 30 Spitfires, or maybe a good ratio of Mustangs to bombers than many bombers with only a few Mustangs to escort them. As far as Battle of Britain was concerned the chain home system made the aircraft so much more effective, it was certainly worth a squadron or two.

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