Implement windows and forms authentication in asp.net mvc 4

I am working on asp.net mvc 4 application. Currently the application is forms authentication enabled. But now i want to implement windows authentication in same application. The scenario is- When application will run, it will by default check for window authentication and once that is authenticated, depending upon the role of user, I need to enable forms authentication.

Following is the setting in web.config-

<system.web>
<compilation debug="true" targetFramework="4.5" />
<httpRuntime targetFramework="4.5" />
<authentication mode="Windows">
</authentication>
<authorization>
  <deny users="?" />
</authorization>
</system.web>

and following is the code in Login Controller-

    [AllowAnonymous]
    [HttpPost]
    public JsonResult JsonLogin(User model, string returnUrl)
    {

        var user = HttpContext.User.Identity.Name;
            if (ValidateUser(model))
            {
                FormsAuthentication.SetAuthCookie(model.LoginUserID, false);
                return Json(new { success = true, redirect = returnUrl });
            }

        return Json(new { errors = GetErrorsFromModelState() });
    }

but i am getting User.Identity.Name = ""; What am I missing? I want to run application on local system itself. Please tell me how to implement this scenario. Any help is appreciated! Thanks in advance!