C# - IIS Overriding Custom 404 Error Page

I am trying to create a 404 error page and currently I have all of the following/tried all of the following to try and accomplish this. When the user types in :

http://name/something.aspx

It works just as its supposed to. But if the user types in:

http://name/NotAFile

with no .aspx then IIS7 takes matters into its own hands and I get the lovely error page that IIS7 comes with. The goal is that the site redirects with only a 404 status code (so not a 200, or a 302 redirect). I have tried in both the web config with:

<customErrors mode="On" defaultRedirect="~/error/Default.aspx redirectMode="ResponseRewrite">
<error statusCode="404" redirect="~/error/NotFound.aspx" />
</customErrors>

This works for the url with a .aspx file extension but not for no extension. Same with this approach in the global.asax

void Application_Error(object sender, EventArgs e)
{
var serverError = Server.GetLastError() as HttpException;
if (serverError != null)
{
if (serverError.GetHttpCode() == 404)
{
Server.ClearError();
Server.Transfer("~/error/NotFound.aspx");
}
Server.Transfer("~/error/Default.aspx");
}
}

The same results are present for this :( My final attempt was to apply this to the web config:

<system.webServer>
<httpErrors existingResponse="PassThrough" />
</system.webServer>

With this I just get a plain white screen with nothing on it...