Google Takeout is one of the service from Google to export/download an archive of data from Google Products. This service was established on 2011 and it supports 10 Google products like bloggers, bookmarks, contacts, etc.,
Google on December 2013 Announced that they are making Gmail data to download as part of Google Takeout Service. This is reported in Gmail official blog. As said now Google enabled Gmail data to download from Google Takeout.
Navigate to Google Takeout. Login with your Google id and start download your Gmail data and you can use it as a backup for your mail.