Correct answer is... 487 . It is equal to 263 kn or 303 mph. It is higher value then max speed of other aircrafts of a similar size. Average is about 337 km/h (182 kn or 210 mph).
The maximum speed most frequently is a term for so-called never exceed speed. Above this speed, the Northrop X-21 can suffer considerable damage. There are also three other types of speed that can be called "maximum speed": Vno (normal operation speed) is the maximum allowable speed of normal operation, Vb (design speed for maximum gust intensity) is the maximum speed in rough air, and Va (design maneuvering speed) is the maximum speed at which the rudders can be maximally deflected.
Disclaimer: Our articles and data connected to max speed of Northrop X-21 are based on public data from various sources. We are very picky to select an reliable data providers, but we cannot guarantee that everything is 100% error-proof. We recommend for any serious purpose - as a rule of thumb - to request offical data about max speed from manufacturer of Northrop X-21.