Why is the letter 'D' missing in the output?

#include <iostream>
using namespace std;

int main() {

    char ch1 = 'A';
    char ch2 = 'B';
    char ch3 = '\n';
    cout << ch1 << '\t' << ch2 << ch3;
    cout << 'C' << '\t' << 'D' << '\b' << ch1 << ch3;
    //return 0;
    system("pause");
}

Output is:

A        B
C        A

Why is the last letter A and not D?