A winning software project proposal needs to address two concerns. The first is whether the proposal describes a credible solution that solves the given problem of the client. The second is whether the proposal has successfully addressed the proposal evaluation criteria of the client.
A software project proposal is accepted by a client after an evaluation process. If one wishes to create a proposal that has high chance of being accepted, the proposal needs to be created in such way where it would successfully address the evaluation criteria. Since the evaluation criteria often differ from client to client, from time to time and from project to project, a proposal writer needs to spend some effort to identify what those criteria are for the given project so that they could be addressed well.
The evaluation criteria often include many checks in addition to evaluating that the proposed solution actually solves the given problem of the client. That is why a winning proposal needs to specifically address the second concern mentioned above.
Even if a proposal articulates a credible solution for a reasonable fee, proposal evaluators often needs to be satisfied with two important aspects of the proposing entity - whether it is a company or an individual. The first is if the proposing entity can deliver the proposed solution. The second is if the proposing entity will deliver the proposed solution.
Hence, if you are to write a proposal that has a high chance of being accepted, strive to propose a credible solution and also attempt to address the proposal evaluation criteria which may not be fully visible. Even in a case where you may not have greater visibility into the proposal evaluation criteria of the client, preparing a proposal that successfully demonstrates that you can diliver and you will deliver has significant merits.