How do I find a formula for a bell curve distribution over a certain amount of months?

We are creating a Revenue Forecast Projection spreadsheet to show our companies sales in the future months/years. We have the Contract Amounts by Job & the duration of the Contract(by month). Currently we are just dividing Duration by Contract amount for the specific period of time and entering that value in each month. We know that our monthly billings start slow and peak, and then slowly decline, like a bell curve. Is there a formula or template we could use to help?