Knowing what was going in the world has always been a focus to build better, faster and high flying airplanes. In an age of satellites, GPS and Google Maps it’s easy to lose sight of just how difficult it used to be to know what was going on beyond the horizon. It was a particular problem for US military commanders fighting Japanese forces in the Pacific during the Second World War. They simply had no means of knowing what the enemy was up to. As early as 1943 there were plans to use long-range bombers flying out of India and China against the Japanese mainland, but they had no reconnaissance aircraft with the range to get there and back, nor the speed and high-altitude capability necessary to stay out of trouble if they could. So they asked US manufacturers for a machine that could do it all. In response, Republic Aviation, the company behind the butch-looking P-47 Thunderbolt, came up with one of the most beautiful aircraft ever designed. Perfectly streamlined and elegantly proportioned, Republic’s XF-12 Rainbow looked like a step into the future. Its performance too was every bit as good as it looked. Hauled through the air by four hugely powerful Pratt & Whitney Wasp Major turbo-supercharged radial piston engines each capable of generating 3500 horsepower, the Rainbow could outrun most contemporary fighters. The trouble was, when the prototype Rainbow was revealed in December 1945, the USAAF no longer had the same pressing need to spy on Japan. Fortunately, the maker had designed it with a view to converting it for commercial air travel. After all, what could be more prestigious for an airline than flying what was, in effect, the Concorde of its day? Both Pan Am and American Airlines placed orders, but in the austere times that followed the war, passengers and airlines were prepared to forgo speed and exclusivity, so bigger, slower, cheaper rivals, such as the Douglas DC-6, cleaned up. Not a single Rainbow airliner was ever built. The XF-12 remains the fastest four-engined piston-engined aircraft ever built and the only one capable of exceeding 450 mph in level flight. But just like Concorde she turned out to be the wrong bet.