App that runs in background & can make itself topmost

I need an app that can make itself topmost after a set amount of time has elapsed. The app will have a button on it and when its clicked will make itself minimise and run in the background. At the same time, it will set a timer of 40 seconds. After the 40 seconds has elapsed, it will force itself to be the current and topmost application. It should still become topmost if the user selects it after double-tapping the home button (standard iPhone app behaviour). Ideally, if the app was ever closed by the user, it should restart itself and run minimised for another 40 seconds before making itself topmost again. I'm not to concerned with what is displayed on the screen in this app. Maybe a black screen with an iPhone logo will do for now. I will have this changed later.