The Triumph Thunderbird is a British motorcycle

Motorcycle

A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...

 that was introduced by Triumph in 1949 and produced in its original form until 1966. The name was used three more times for new and distinct Triumph models.
To capture the American market, the 6T Thunderbird used a variant of the earlier Speed Twin's parallel twin engine, bored out from 500 cc to 650 cc to give the added horsepower American customers demanded.
The concept of enlarging the Speed Twin, the Thunderbird name and its 'paper dart' logo were thought up by managing director Edward Turner

Edward Turner

Edward Turner was a British motorcycle designer. He was born in Camberwell in the London Borough of Southwark, on the day King Edward VII was proclaimed King....

 on one of his regular trips to Triumph's operations in the USA.