How to insure "B" booking code
 

How can you make sure that what you're booking is "B" code on United. I see the list of available codes in the flight details but there are many.

I don't think there is any way to force a B fare; if you select "Refundable" it will give you the LOWEST refundable. If you want a specific fare you can probably call web support and ask that they do it (to avoid the phone fee).

If they won't/can't then you'd have to call to book it as far as I know (or use a TA).

I just picked "refundable" on some random dates, IAD-SFO and got a "M" and and and "E" fare:

http://img138.imageshack.us/img138/2074/bookingcode.jpg

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Though I am not an expert with UA website, you can try this:
When you are booking a ticket, select the "YB Upgrade" button. In that case, it will show you Y or B fares.....I have not had 100% success with this method. But it is one option.

call ua res....

A travel agent can do it for you.