I've opened an account with a US passport. A couple of points worth noting in my experience:
1. During the ID verification process, it was a little challenging for the agent to scan the specific security features of the passport (e.g. holograms and color changes). This may be because my smartphone has no flash. YMMV.
2. In order to pair your smartphone with your account, within the app you will prompted that a security code has been texted to you and you are to enter it in the app. But no matter how many times I tried, I never got the text. Finally called N26 support and was informed that their texts don't go through to US/Canadian numbers. So, he gave me the code over verbally. Then it worked. Would be nice if they had some mention of that on their support pages.
So, now I have the account and I'm baffled by how I can actually deposit money it while I'm still in the US so that I have money in the account upon arrival and to purchase a BahnCard Flex (replacement of the old Probe BahnCard, which requires a SEPA account). Can I just send a direct wire transfer from US bank to my N26 account? What about PayPal? Any advice greatly appreciated.