Many people have been told the Executive Summary should be a one-page document prepared after the proposal is written. This is unhelpful advice as executive summaries are the most important pages in your proposal.
A draft executive summary should be prepared early during the capture planning or pursuit phase, well before the RFT, providing a roadmap for capture.
The BD Manager should prepare the first draft of the executive summary and present it at the Preliminary Bid-Decision gate review. Executive support for capture investment should be based on the executive summary as well as more traditional factors.
Over time you can use the executive summary to test your assumptions about your customer with them. Their feedback should be the basis for evolving updates through to your eventual proposal.