The first thing the Royal Air Force said about the P-51 Mustang wasn’t praise. It wasn’t criticism either. It was a warning.

In the autumn of 1941, a cargo ship arrived in Liverpool carrying an aircraft inside wooden crates. According to one account, the ship was bombed while crossing the Atlantic, but the crates remained intact. Inside was a single-engine fighter from a California company almost no one in Britain had heard of.

Mechanics assembled it, fueled it, and prepared it for flight. Before it took to the sky, the gunners had to be warned. The new American fighter looked remarkably like a Messerschmitt 109 from the ground. Squared-off wingtips, a long slender nose, a tight streamlined profile. To anti-aircraft crews who had spent over a year learning to shoot first and ask questions later, that silhouette meant enemy.

So orders went out to the batteries and fighter stations. This aircraft is ours. Leave it alone. Soon, many frontline Mustangs carried bright yellow bands across their wings so British pilots wouldn’t mistake them for German planes in the chaos of battle.

That was the beginning. Over the next three years, a remarkable number of RAF pilots climbed into this aircraft for the first time and climbed out with a private opinion. Test pilots and squadron pilots. Men who learned their trade on biplanes, and men who had fought the Battle of Britain in Spitfires. Some of what they said was dismissive, some was wrong, and some seemed at the time like pure fantasy.

This is the story of those judgments, in the order they were made. One of those opinions was written down the morning after a single half-hour flight by a man who had never fired a shot in anger. That opinion would put a British engine into an American airframe and change what was possible in the air war over Europe.

His employer’s reward for that opinion was a raise of one pound sterling a week.

To understand why so many experienced pilots misjudged this aircraft, and why one man got it exactly right, we have to start with the reason it existed at all.

Nobody asked for it.

In the spring of 1940, Britain wasn’t looking for a masterpiece. It was looking for anything with wings and guns. France was about to fall, and the RAF couldn’t build fighters fast enough for what was coming.

In New York, the British Purchasing Commission was combing through the corridors of the American aircraft industry, trying to buy anything that could be delivered soonest. What the British really wanted was more Curtiss P-40s, a sturdy and unexciting fighter already in production. Curtiss had no additional capacity.

So the Commission turned to North American Aviation, a young company in Inglewood, California, whose most famous product was a trainer. The RAF knew that trainer well. They called it the Harvard, and thousands of British pilots had learned to fly in it.

The British proposal was direct. Build us P-40s under license.

North American’s president, a blunt former Army pilot named James Kindelberger, known to everyone as “Dutch,” refused. He argued that tooling up to build another company’s fighter would take as long as designing a better one. His engineers would give Britain a completely new fighter of their own design, and quickly. The British accepted the gamble, because they had no other choice.

The design team, led by Raymond Rice and an engineer of German descent named Edgar Schmued, worked in a smoke-filled room directly above Kindelberger’s office. Sixteen-hour days were normal. One of them later recalled that Sundays felt different only because the neighboring factory observed quiet.

The heart of the design was a gamble on the wing. North American chose a new laminar-flow wing, based on research from the National Advisory Committee for Aeronautics, the American government’s flight lab. The idea was to keep air flowing smoothly over the surface for much farther than on a conventional wing, dramatically reducing drag. No one had ever trusted such a wing on a mass-produced fighter.

When the first wind tunnel results came back, they seemed wrong. The engineers suspected the tunnel itself was too small and was spoiling the airflow around the model. So they loaded the model onto an airliner and flew it north to a larger tunnel in Seattle, where it behaved exactly as they hoped. Before the Mustang ever flew, its wing had already flown as a passenger on another aircraft.

The first airframe was rolled out 102 days after the contract was signed, while it was still waiting for its engine. On October 26, 1940, a freelance test pilot named Vance Breese lifted it off from Mines Field in Los Angeles, the ground beneath today’s international airport. That December, the British gave it a name. They called it the Mustang.

Rapid production followed. The first Mustang built for the RAF flew in California in April 1941, and by that autumn, the crates were crossing the Atlantic, bound for Liverpool and the gun crews who had to be told not to shoot.

But one choice in all of this mattered more than any other, and at the time, it didn’t even look like a choice. The engine was the Allison V-1710, the same basic powerplant as the P-40, reliable, American-made, and available. The version North American could use had a single-stage supercharger. As the aircraft climbed into thinner air, the engine lost power gradually. At low altitude, it was strong. At high altitude, where the fighter battles over Europe were fought, it faltered.

No one in Inglewood could fix that, and for the next eighteen months, it colored everything the RAF said about its new fighter.

The first detailed British assessments came from people whose job was to be unimpressed by anything. New aircraft arriving in Britain went to RAF test units, where pilots flew them with a stopwatch and wrote down exactly what they found, without embellishment.

There’s a story, later told by North American personnel themselves, that after an early demonstration in England, the performance figures seemed so implausible that British officials refused to believe them and ordered the flights repeated. Whether every detail of that story is true or not, the numbers that followed were real. They were also very strange.

In the spring of 1942, the Air Fighting Development Unit at Duxford pitted a Mustang against a Spitfire V, the backbone of Fighter Command. At 5,000 feet, the Mustang was about 30 mph faster. At 15,000 feet, it was about 35 mph faster. And the Spitfire was making that speed with a more powerful engine. The American plane was going faster on less power, and there was only one explanation. Its design was slicing through the air more cleanly than anything the RAF owned.

Then the testers took it higher, and the picture flipped completely. Above 15,000 feet, the Allison began to weaken. At 20,000 feet, the Mustang’s maximum speed dropped to about 357 mph, slower than the Spitfire V and slower than the Messerschmitt 109F. It was supposed to fight at that altitude. The climb to that height took about 11 minutes. The Spitfire V did it in seven.

That was the assessment that mattered in the end. Fighter Command fought its war at 25,000 to 30,000 feet over northern France. A fighter that ran out of power at 18,000 feet was of no use to it. So the Mustang was politely ignored and handed to Army Co-operation Command, the part of the RAF that worked for the soldiers, doing reconnaissance, photography, and ground attack.

If you were an Army Co-operation pilot in early 1942, this was not a disappointment. It was more like a Christmas present. Consider what some of those men had been flying before. Many of their units had spent the early war with the Westland Lysander, a high-wing, fixed-gear machine designed to land in fields and spot artillery. Others had the Curtiss Tomahawk, the RAF’s name for the early P-40. Now they were being handed a fighter faster at low altitude than a Spitfire.

No. 26 Squadron at Gatwick received theirs first in early 1942. For pilots coming from older types, the leap was enormous. By British standards, the cockpit seemed roomy, with space to move and an American layout that took some getting used to. At low altitude, the engine ran smoothly and strong. The aircraft carried an unusual weight of guns and enough fuel to go a long way and come back safely. Behind the pilot’s seat, the RAF fitted a sideways-facing camera so the pilot could fly along a coast or a railway line and photograph everything he passed.

On May 10, 1942, a pilot of No. 26 Squadron, Pilot Officer G. N. Dawson, flew the Mustang’s first operational sortie, crossing into France to strafe aircraft hangars and attack a train. By midsummer, eleven squadrons had Mustangs. The work they did was intimate. Mustangs crossed the Channel in pairs, sometimes alone, flying low enough to stay under German radar. They chased locomotives, barges, and trucks. They photographed beaches, bridges, and gun positions the Army might one day need to know about. The idea was to be fast, low, and gone before anyone could respond.

And down there, at 100 feet over the fields, the Allison engine that had failed its high-altitude test was in its perfect element.

So what did those pilots say about the Mustang? Mostly they said it to each other in crew rooms, mess huts, and canteens. What survives on paper is very little. Lines in squadron operational record books noting takeoff time, landing time, target, and result. A locomotive damaged near a French town. A photo mission along a stretch of coast. One aircraft missing. Hundreds of similar sorties were flown in 1942 from a handful of airfields in southern England. Almost none of them were filmed or told to anyone outside the squadron.

And so most of the detail of this war happened in reality. Not only in the famous battles, but in small, repeated acts by people no one filmed. One airfield among hundreds, one sortie among tens of thousands. The same kind of story repeats again and again in places no camera ever reached.

There are far more stories of this kind than any one channel could contain. Eleven of them existed here, told in full, until I removed them myself. Now they exist in only one place, an audio collection called “Behind the Wire.” Eleven complete stories from the war, nearly 6 hours of narration, and a companion booklet to keep beside it. You can’t watch it anywhere anymore, but you can still hear it. The link is in the description.

Ten days before Dawson crossed the French coast, a very different kind of pilot had already climbed into a Mustang at Duxford. His name was Ronald Harker, and he belonged neither to the RAF’s test establishment nor to a fighter squadron. He belonged to Rolls-Royce.

Harker’s report landed on the right desks at the right moment. Within weeks, the Merlin 61 was mated to a Mustang airframe at Hucknall, and the machine that emerged was no longer the aircraft the RAF had dismissed. The first flight, on October 13, 1942, was not a test of whether the idea would work. It was a confirmation that the men who had pushed it had been right all along.

The numbers that followed silenced every doubter. At 22,000 feet, the modified Mustang touched 433 mph. Its ceiling climbed past 40,000 feet. The fighter that had been declared useless above 18,000 feet was now faster at altitude than the Spitfire that had lent it its engine.

By early 1944, Mustangs were escorting American bombers all the way to Berlin and back. The “Little Friends” no longer turned for home early. They stayed, and they hunted. What began as a warning to gunners not to shoot became a warning to the Luftwaffe that the sky had changed hands.

Ronald Harker flew a half-hour sortie, wrote a report, and accepted a raise of one pound a week. He never fired a shot in anger. Yet that single opinion, written on May 1, 1942, put a British engine into an American airframe and altered the course of the air war.

So here is the question worth asking: how many world-changing ideas are sitting in a report nobody has read yet, waiting for one person to look at the same numbers and see something different? What do you think?