It is now required for every company to have a D-U-N-S number in order to have an Apple Developer account.
Existing accounts will eventually need to provide one, we would guess at least when they renew their account. Since this can be a lengthy process we recommend that you apply for one as soon as possible.
Read more about it on Apple's developer site: https://developer.apple.com/support/D-U-N-S/.
Apply on D&Bs website.