What did I see?

I was driving eastbound on a familiar road.  It went straight as a die for many miles and had a low horizon of bush.  The day was overcast, in early fall.  As I drew closer to the perceived line of trees, there was an object in the sky slightly above the horizon.  It resembled nothing else but a huge black kite, and, indeed, it behaved like one, in the respect that it gradually changed positions in the sky.  Sometimes viewed almost edgewise, other times as a definite kite shape.  What struck me was its size.  I knew it had to be immense, because the further I approached, it did not seem to grow larger or change positions relative to me,  a sensation very similar to trying to drive by the moon.  It stayed with you.  I viewed it for a full five minutes before I had to cut off towards my destination, for which I was already late.  I regret that now.  It was definitely changing shape or moving obliquely in the sky, and presented different aspects during this time.
It had a stark outline, with straight sides, and could not have been a cloud.  This mystery has stayed with me for the twenty some years since I witnessed it.