Each hero has a specific requirement to obtain.
Here is the list:
Hans-Ulrich Rudel
You have to deal 70+ damage* with your tactical bomber to land or naval enemy units and use Air recon HQ skill 10+ times.
Can serve in: tactical bomber.
Kurt Knispel
You have to deal 70+ damage* with your tank to the enemy tanks, recon units, anti-tanks (only damage to these types of units counts).
Can serve in: tank.
Michael Wittmann
You have to deal 50+ damage* with your tank or self-propelled anti-tank to the enemy tanks, recon units, anti-tanks (only damage to these types of units counts).
Can serve in: tank, self-propelled anti-tank.
Rudolf Witzig
You have to deal 60+ damage* with your infantry unit to the enemy forts and coastal batteries (only damage to these types of units counts).
Can serve in: infantry.
Fritz Christen
You have to deal 20+ damage* with your anti-tank unit to enemy units(any) while this unit (your anti-tank) has 4 or less strength (In the next update we plan to increase it to “6 or less strength”).
Can serve in: anti-tank.
*The damage has to be done with a single unit across any number of missions. Each of your units of corresponding type has its own counter, so they all work towards the final goal independently. However, once the first unit reaches the requirement, you gain a hero and all counters for this hero are frozen.