Without knowing the rest of the details of your house, you really do need to reframe that window.
The way it is framed now will likely cause the header to fail. What will happen is the header will put outward pressure on the studs causing them to bow out, pulling out the nails and allowing them to bend. If you are going to forego jack studs you need to use hangers for the header. The exception to this is if the window is on the gable end on the floor directly below the attic, and the roof is framed with trusses, and the end truss is designed to allow it.
Framed properly the footer is completely unnecessary. Worse case is the sill will only carry the weight of the window, and a 2x4 on 24" on center cripples will carry that weight most of the time. I would argue that the footer and over-sized header are actually doing more harm than good. That is because they massively increase the un insulated area of the window. A small decrease in the insulated area causes an outsized effect on the total r-value of a wall.
4 Types of R-Value Typical wall construction causes about a 50% loss in whole wall r-value, and that's not even counting the R-value loss from the windows.