As for the flying car, apparently one of the problems the military has with their cars is that roads offer the enemy a guide for where the vehicles are going. (Maybe they should follow the slugs.) As DARPA explains:
DARPA said the TX will "provide the flexibility to adapt to traditional and asymmetric threats by providing the operator unimpeded movement over difficult terrain. In addition, transportation is no longer restricted to trafficable terrain that tends to makes movement predictable."
As for the underwater plane, don't call it a flying submarine:
Announced last year, the agency's Submersible Aircraft research project is exploring the possibility of making an aircraft that can maneuver underwater with the goal of revolutionizing the US Department of Defense's ability to, for example, bring military personnel and equipment to coastal locations or enhance rescue operations. DARPA said that the concept being evaluated here is for a submersible aircraft, not a flying submarine. It is expected that the platform will spend the bulk of its time in the air and will only spend short periods of time submerged according to the agency.
According to DARPA: "The difficulty with developing such a craft comes from the diametrically opposed requirements that exist for an airplane and a submarine. While the primary goal for airplane designers is to try and minimize weight, a submarine must be extremely heavy in order to submerge underwater. In addition, the flow conditions and the systems designed to control a submarine and an airplane are radically different, due to the order of magnitude difference in the densities of air and water."
I love the idea of flying cars (though I don't care so much about the underwater plane), and these DARPA competitions are a great use of incentives. Maybe the future really is up in the air.