The device, removed from a UPS aircraft by Leicestershire police officers shortly after 3.30am on 29 October, was timed to go off some seven hours later, the Yard said.
A spokesman said: "If the device had not been removed the activation could have occurred over the eastern seaboard of the US."
The bomb, disguised as an ink cartridge, was found following a tip-off from Saudi intelligence.
A Scotland Yard spokesman said the cargo plane arrived at East Midlands Airport from Cologne at 2.13am BST and left at 4.20am BST "after the suspect package had been removed".
He added: "Forensic examination has indicated if the device had activated it would have been at 10.30am BST."