X-313 "SpeedDemon" - The Kerbal Griller

But Kraken damned hot while flying, the cabin's internal temperature reaches easily 400°K (~126°C or ~260°F) with less than 5 minutes of flight at Mach 3.7. But it comfortably flies around Kerbin without sweating (the plane - the poor pilot, on the other hand, ends the endeavour soaked in sweat and pretty well done).

She's pretty stable on flight, tough.