Can't downcast because class is not polymorphic?

Is it possible to have inheritance with no virtual methods? The compiler is saying that the following code is not polymorphic.

Example:

class A {
public:
    int a;
    int getA(){return a;};
}


class B : public A {
public:
    int b;
    int getB(){return b;};
}

In another class we are trying to downcast from an A object to a B object:

 A *a = ...;
 B *b = dynamic_cast<B*>(a)

but this gives the following compile-time error:

 cannot dynamic_cast ... (source type is not polymorphic)