Somewhat in the spirit of the way the Air Force did this with aircraft designations when they started getting into high numbers.
...It has to be remembered that the low numbers had never been used in the scheme in the first place. When the Air Force was founded, the old USAAF numbers still in use were "adopted" into the new scheme, with Pursuit planes like P-40 Warhawk becoming Fighters with F designations. And AFAIK, the lowest-numbered such designation theoretically adopted was P->F-38 Lightning, while "the highest of the low" the modern USAF has used so far is F-35 for the JSF (coincidentally named Lightning II). Even F-38 was never used in practice, though, the "lowest high" F-fighter in USAF service being F-80 Shooting Star. There's a long way to go till they hit that number!
The old USN designations are a different matter entirely...
The old USN designations are key to the story. When SecDef McNamara decreed in 1962 that aircraft designations be uniform across the services, the navy system had to be fit into the USAF/army system. To make it as painless as possible, the navy planes were given numbers similar to their old numbers. So the F3H Demon became the F-3, F4H Phantom II became the F-4, the F8U Crusader became the F-8, and so on. But, this didn't always work out, so the F3D Skyknight became the F-10, and the F4D Skyray became the F-6.
At the time, the USAF was designating the Phantom the F-110 (and calling it Spectre), but it in the new system it became the F-4. So the USAF fighter numbers got re-routed back to the low end. Except for the F-117, which is a story all its own.
--Justin
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