I haven't done it, but I think adding about a dozen overripe black bananas (per 5 gallons) late in the fermentation would work wonders for real banana flavor.
I also have a suggestion for peanut butter flavor -- toasted Maris Otter. Put some Maris Otter in a 350 F oven for 15 minutes. You'll get a ton of nutty flavor in the final beer.
I do like the hefeweizen yeast idea too. Ferment warm around 70 F for maximum banana. With a big pitch and warm temps, you shouldn't get much clove. WLP300 *should* be fine for this although I'm still experimenting with weizen yeasts myself so I can't say for sure. Wyeast 3068 is definitely known for being a banana bomb.