Call-for-artists fall into two catagories typically:
Request for Qualifications (RFQ's), and
Request for Proposals (RFP's)
Then there are also Slide or Artist Registries that local public art commissions maintain.
RFQ's are when the first round of selection is based on slide portfolios and resumes (previous experience). The Selection panel usually does not want to see proposals for these, though may consider concepts. RFP's are when the selction will be based on both previous experience and the proposed concept, usually with drawings or renderings. Slide registries are lists of artists and examples of their work that are kept on file and reviewed when money comes up for purchases. These are usually smaller purchases (usually 2d or small 3d), often for pre-existing work.
When you download a Call-to-artist, it will list exactly what is needed.
As Glenn says, its a hard nut to crack, with luck as much a part of it as anything. For those looking to increase the visual punch of their RFP's or semi-finalist proposals, I offer 3d rendering services that are really pretty affordable for what you get. Take a look at the projects page of my website
to see the kinds of imagery (still and animated) that can be produced with 3d scanning and 3d modeling. As Glenn also said, as strong presentation can be essential, and a strong presentation of a strong concept can be especially convincing.
Also look at my SERVICES link to see the specific services I can offer. I can scan a rough clay model, surface it in the final materials desired, light it and site it, and show multiple views or fly-by animations. The images below are of a proposal I worked up for Paula Slater for a project she was up for-- she didn't get this one, but she said she came close. This model was developed from concept drawing and photos of her clay maquette (which were also included in the proposal-- she didn't have time for me to scan it so I modeled the figure from the photos of her clay).
Good luck with it,