Generally speaking, yes, it's possible to get money from USA investors even though your company is not local.

On the personal note, I am glad to see that all your programmers are from Serbia, as that's my home country and I live in the US now.

The app does not have to be done at all. 60% is good enough to test the market, so I would shift my focus on product--market fit validation, customer acquisition process and a business model with an emphasis on revenue potential, addressable market size, total market size.

Seed investors will put their money into pre-revenue companies, but venture capitalist, Round A are less likely to do so which makes bootstrapping incredibly important.
Here's my paper that can help you understand where does your company fall on investment readiness spectrum.
How do you know that #startup is ready for venture capital? Industry fit, $1B+ market size, big competitive advantage http://wp.me/pj2e9-by

Finally, I would be more than happy to work with you on your investor's pitch and be ready before your colleague starts making connections in SF:
http://www.anagard.com/services/InvestorReadyConsulting.html