try
ui->menuBar->hide(); // hide menubar
ui->menuBar->show(); // in case you want to restore it
in your mainwindow class if you want to hide it in the runtime.
Otherwise, you don't have to create a project with mainwindow.ui everytime. As you start a new project, there's an option in [Details]->[Base class] which allows you to choose the base class (template form) as mainwindow, dialog or widget.