Set the autoenter options (File>Manage>Database, doubleclick on the fieldname) to autoenter "B".
Then set the field Behavior to allow entry in browse mode.
(This will only give you headaches if you additionally have a validation on the field...so think through any validation you want on the field, if any)
If you are already autoentering a serial number in this field (unclear from your post), simply set the autoenter serial counter to "B000001" and increment by 1. If this is, indeed, the case...you'd be better off having your field type set to text rather than number.
Additionally, if this is really an autoenter serial used for a Unique record ID, you should not edit it or overwrite it...again, I can't tell from your post how you are using this field.
Thanks for your help ninja