The Superbird was the Plymouth response to Dodge's 1969 Daytona. Same idea: nose cone, tall rear wing, slipstream-cleaned body — but on a 1970 Road Runner shell rather than a 1969 Charger. Plymouth's specific motivation was to bring Richard Petty back from Ford, where he had defected for 1969; the wing-car was part of the deal.
Production: per Wikipedia, the generally accepted figure is 1,935 Superbirds built and shipped to US dealers (with 34–47 additional cars allegedly shipped to Canada). For 1970 NASCAR raised the homologation rule from 500 examples (the 1969 rule that the Daytona barely cleared at 503) to one car for every two of a manufacturer's US dealers — for Plymouth, that meant a 1,920-car minimum. The engine breakdown most often cited: 135 Hemi Superbirds, 716 with the V-code 440 Six-Barrel, and the remainder (≈1,084) with the 440 4-barrel.
Designer — the underlying body was John Herlitz's-era Plymouth Studio work on the 1968 B-body (shared with the Belvedere / Road Runner line). The Superbird-specific aero — nose cone, wing, flush rear window, vinyl top to hide the leading-edge weld of the rear-window plug — was engineering work led by John Pointer's team that had developed the Daytona, applied to the Plymouth body with Plymouth-specific tooling. NASCAR rule changes for 1971 made the wing cars uncompetitive (the 5-litre engine cap killed the Hemi's advantage, then the 1971 banning of the wing cars closed the chapter); both the Daytona and the Superbird were 1969 + 1970 only.