We need to develop more-complete and more-accurate specifications.
What is the rest of the load spec? i.e. what is the possible range of the resistive component of the load impedance?
And what input voltage would you be seeing, when the output would need to be 300 mA? OR, what is the Gain factor it needs, i.e. Vout/Vin?
You originally said more than +/- 40V and more than 300 V/us. Assuming Class AB, if one transistor polarity had to slew the output from 0 to +40 at 300 V/us, that would imply 40V /300 V/us = 0.133 us rise time, which would imply f = 1 / (3.14 x 0.133 us) = 2.39 MHz.
Also, To get 0.3 Amp DC at 40V, the load resistance would need to be R = v/i = 40/0.3 = 133 Ohms or more. How does that compare with what it might really be?