It is not Fee Simple Determinable because the property does not vest in B and his heirs until the contingency occurs. If would be a defeasible estate if it said "To B and his heirs so long as..."
You see the difference? In a defeasible estate, B gets the property but it can be taken if the condition is not met. In your example it is the reverse. B is not vested until the condition occurs.
Forgive me if I used the wrong buzz words, its been a while since property.