Redirect to new domain that serves new and different content

Welcome to SO. You’re redirecting the users of old website to a new website which will not send the content to the new site. In order to do that what you can do is install a plugin, I usually recommend Updraft plugin to transfer the content of old site to the new site.

This is not only easy to use but also helps in transfering everything including media and database from the site a (old site) to the new site (site b).

Edit:
Update your .htaccess to something like this:

Options +FollowSymLinks
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^OLDDOMAIN\.com$ [NC]
RewriteRule ^(.*)$ http://NEWDOMAIN.com [R=301,L]

Hope this answers your question.