Two new clues, plus an addition to the first clue:
Looking for the name of a warbird.
1. One of the very first planes of its type to be designed for use in a particular role. Possibly the first, but I'm not at all sure. It was, I'm fairly sure, the first plane of its type to be used in that role.
2. Fewer than 20 built.
3. It was used for only a couple of years by the one service that had purchased it. One foreign country bought one of them; whether it ever used it for anything but testing I don't know.
4. It had many good qualities, but it was somewhat underpowered. That weakness was enough to end its military career.