This paper argues that financial intermediaries serve to overcome competitive externalities between investors. A borrower has access to a long-term project that is subject to an uncertain intermediate date refinancing need. Investors compete to offer financing contracts but cannot control or verify the firm's contracts with other investors. If the firm deals with each investor bilaterally, this leads to a double common agency problem: on the one hand it becomes difficult to limit the maximum refinancing ex ante. On the other hand, when multiple investors contract with the firm at the same time, they want to make sure the other investors supply the refinancing. An intermediary arises naturally to bundle various investors' resources and unilaterally deal with the borrower. This can explain why certain banking models, such as universal banking, investment banking, or syndicated lending emerged as the way to finance long-term industrial investment with large uncertain refinancing needs.