I know many people that have been in similar situations. If you just have a concept and cannot build the app yourself, I recommend you hire a dev team and bootstrap it yourself or with the help of friends and family. There are not many investors out there anymore that are willing to invest in just an idea (there are just too many alternatives for them). Another option that you can consider is to find individual developers or a company that is willing to build the app for you in exchange for equity. However, this is not as easy as it sounds because of the risks that the developers have to take (unless you know some developers personally). Also another issue I see here is the fact that you don't want to share any information about your app. Its difficult to get people interested or receive feedback this way.
In any case, the ideal order of action would be :
-Get developers to build a test version
-Have it tested by users and receive feedback
-Improve app by the feedback you receive
-Launch final version
-Approach investors for funding
Hope this helped.