You basically tell DnB that you are applying for a US government contract. They are then required to give give you the number within 1 day.
They will tell you that they will revoke your number if you don't enter yourself in some government database within 90 days. You can then enter yourself there (or as happened for us the first time we needed the number, we were just too busy to do that and figured that we would let it expire and deal with the consequences if anyone really needed to use the number).