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:



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