I have no great knowledge of your climate or of growing peanuts but they need a soil temp of 65 -70f to germinate and I'm wondering if you have that early enough in your region?

To fix nitrogen you can use any member of the Fabiaceae (pea family). Favas might be more suited to your climate. And you get to eat the beans too. Other options are clovers or any beans and peas.

I would say by May 1st our soil is 65 degrees maybe as much as a couple weeks earlier. Our last snow is usually in early March.

farmerdill - what are the prospects if the OP only wants the N fixing nodules and isn't bothered by getting actual peanuts. (I don't know if that's the case). Would he stand a chance of getting decent plants?

It does not make sense to grow peanuts solely for their nitrogen production. It is far better to grow them to make peanuts. The early spanish peanuts are 110 days to maturity. The Black Peanut sold by Sandhill is also 110 days. There is a short season red peanut that I saw a few weeks back that is also rated at 110 days or less. I don't have the website linked, but a bit of searching should find it.

Concur. If nitrogen fixation and green manure is the desired result, I would recommend clovers Crimson, Red, Alsike or such. Peas (P. sativa) also work well especially Canada Field peas .