First of all, the first year you cannot withdraw. Between Year 2 and Year 5, when you withdraw you lose 3 months of interest. After year 5, you do not lose anything when withdraw.
Only PAPER I bond needs to go thru the bank. Electronic I Bond can be bought and "sold" from Treasury Direct account.
Here is the answer on Treasury Direct website
Where can I bonds be redeemed?
If you own electronic I bonds, you can redeem them in the TreasuryDirect application. If you own paper I bonds, you can cash them at some local financial institutions or by mail.
Here is the FAQ link
https://www.treasurydirect.gov/indiv/research/indepth/ibonds/res_ibonds_ifaq.htm#redeemed
