In a level turn, the aircraft must generate a centripetal force to stay on its circular path. The centripetal force is provided by the horizontal component of the lift. The equation for the bank angle \( \theta \) is given by:
\[
\tan \theta = \frac{v^2}{g \times r},
\]
where \( v = 100 \, \text{m/s} \) is the velocity, \( g = 10 \, \text{m/s}^2 \) is the acceleration due to gravity, and \( r = 1000 \, \text{m} \) is the radius of the turn.
Substituting the values:
\[
\tan \theta = \frac{(100)^2}{10 \times 1000} = \frac{10000}{10000} = 1.
\]
Thus, \( \theta = \tan^{-1}(1) = 45^\circ \).
So, the bank angle is:
\[
\boxed{45^\circ}.
\]