When I was decided about the overall design, I created the plane in maya. Often I keep the drawing very rough and sketchy. I consider it more of a tool to find good shapes and try out ideas as opposed to make great drawings. In maya it's works fine to play around with proportions and see how everything works in 3D. For some parts I create some smaller sketches or create a paint over from a screengrab in maya.
In case your wondering what the pictures of all the donkeys are about. It's about the baggage that's strapped on them. I used that for the baggage strapped below the wings.
The color image is an paint over of the 3D model. Here are some of the small sketches I created: