Daytime sighting of black craft

I deliver propane to rural areas in the San Bernardino mountains. While I was filling a customer’s tank I observed a small dark speck directly under the sun and over Santa Rosa mountain. I tried to focus on this small speck to see if it was moving but it appeared to be stationary.
I was thinking to myself that whatever this object was, I wish I could get a better look at it. The very next moment a large (maybe 40-60 ft long) black rectangular shaped object just appeared directly in my field of vision (in between myself and the mountain). The object was black and extremely bright white light emanated from the bottom. It seemed to me that I was observing the object edge-wise so I can’t exactly say what shape it was but from my perspective it appeared rectangular.
The object moved a short distance and disappeared as suddenly as it appeared.
Even though the close-up portion of this sighting only lasted a few seconds, it shocked and even frightened me a bit.
I’ve had several other night time sightings in these mountains but this was my first day time sighting.