Response.Flush with MVC Result is not working

We are implementing long running process to download database content as xml with asp.net 3.

Total document generation time is about 3-5 minutes and we want to respond with first byte as soon as possible. We have overridden System.Web.Mvc.FileResult and are trying to do something like:

protected override void WriteFile(HttpResponseBase response)
{
   Stream outputStream = response.OutputStream;
   byte[] header = xmlExportService.XmlHeader();
   response.Flush();
   outputStream.Write(header, 0, header.Length);
   response.Flush();
   //// some long running generation here
}

On local iis under windows 7 I am gatting Save File dialog in browser. On production, windows server 2008 R2 there is no dialog until whole file is generated.

Any ideas?