Here's an interesting one...the Bell Model 16.
Originally designed as a twin-boom pusher with the designation XP-52, the model 16 was intended to have a contra-rotating propellor in a pusher configuration driven by a Continental XIV-1430-5 inverted V-12. It was to have two 20mm cannon in the nose and six .50 MG (3 in each boom leading edge).
With the delays in the V-12 engine, Bell's engineers turned to the P&W R-2800-23 engine and started a modified project based on the XP-52, designated the XP-59. So the XP-52 was cancelled on 3 November 1941 and the Model 16 project continued on as the XP-59. Two prototypes were ordered, but the project was cancelled before they were built. The XP-59 designation was eventually given to the P-59 jet project.
Shown is the Model 16 (XP-59) mockup at Bell's facility: