Using MCV4 for returning XML

I am using MCV4, but the what is returns is always JSON, by using "get" function.

I want to return an XML or simple text, and not "JSON" format (which always prompt me to save it).

I have checked webapiconfig:

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );
    }
}

but I didn't realize where to declare that the output is "simple text" (and not json auto converted).

Also - how can I output to xml?

Thanks :)