A pre-launch raise is really hard but not impossible. I raised a seed round for a mobile app that hasn't launched but it was quite honestly incredibly difficult and I wouldn't recommend others trying it!
I answered a question about when is best to raise under what circumstances here: https://clarity.fm/a/3484
I have helped many Clarity members with early-stage fundraising advice and would be happy to talk to you about what makes most sense for you and your company.
It is very difficult to get funding for an app that has not launched yet (there are some exception of course). Your potential investor does not know whether the app has a market or not. Besides most savvy investors are not looking for a product, they are looking for a business to invest in. You have to remember that a single app does not make a company. So, it is better to think in terms of how the company will established and grow its market share. It is better to confirm a product-market fit and gain traction within your target market before asking for funding. This has two advantages: Investor has confidence that app will do well in the marketplace and the company has a future, also you as an entrepreneur you can retain more equity in your app/company.
How close is the app to completion?
If it's almost done you should really do a soft-launch and prove some of your assumptions out (see http://blog.tapstream.com/post/71538606229/the-art-of-soft-launching ).
Otherwise, it takes a strong track record in building successful apps to impress investors, or some very unique domain expertise. The mobile app space is extremely competitive.