From Kerbal Space Program Wiki
The Thunderbird is a high performance jet plane with two external droptanks. It resembles the North American F-86 Sabre.
|“|| The Thunderbird is a muscle-car of an aircraft. Broadly similar in performance to the Osprey, it lacks the Osprey's graceful lines (and abilities as a seaplane) in exchange for a higher top speed. Indeed the Thunderbird is basically an engine with a cockpit and some wings bolted on...and therefore a huge success with the test pilots. Make sure you drop the droptanks when they empty to achieve best speed!