The cash flow feature was focused on solving the customer problem: "Will I have enough money left over at the end of the month to pay my bills?". After going broad with the design team, we settled on a calendar format with an alternate chart view to illustrate different aspects of their cash flow picture.
Mint's users had consistently requested a bill reminder feature so we turned to them for initial details about the specific problems the feature should solve. Through Facebook and data mining we developed a detailed understanding of our users' bills, quantity, frequency, and payment methods. We explored large and small implementations.
Customer testing helped us refine the design to a much simpler implementation. We launched bill reminders as the first test of Mint's "sneak preview" feature, allowing us to turn on new features for only a select few opt-in users. We gathered feedback in-product from these users, revised the feature and launched to the entire Mint user base.