One of the originally planned routes to traverse Pennsylvania, US 240 was a designation which did not last long.  The Pennsylvania Department of Highways was upset with the American Association of State Highway Officials' choice of alignments for the then new United States Numbered Highway System.  While AASHO chose alignments that were most congruent to north-south and east-west travel, the Department of Highways wanted single US routes to replace each of the cross-state PA routes that had been posted just a year earlier which they themselves had replaced named trails such as the Lincoln Highway.  In the case of the latter, US 30 and US 1 were picked to replace PA 1 which was the numerical designation for the famed highway.  Even more confusing was US 111, US 11, and US 120 were set to replace PA 4 along the Susquehanna Trail.

Pennsylvania did not send representatives to the Joint Board on Interstate Highways North Atlantic States meeting nor gave a reason, but threatened to not endorse the plan.  AASHO added several routes to the system prior to November 11, 1926 when the US routes became official at the request of William H. Connell, Pennsylvania's Engineering Executive and Deputy Secretary of Highways but even still, it was not enough to appease the Commonwealth.  By early in 1927, issues of state line connections had been resolved so something else was stopping the Commonwealth from agreeing to posting US route markers.

More changes to routes continued to take place throughout 1927 involving the alignments of US 19, US 120, and US 422 inside the state.  The section from Frederick, Maryland north to Harrisburg had been dropped by the AASHO route log in 1927.  By the time things were ironed out and route markers were actually posted in the Commonwealth, US 240 was replaced with an extended US 15 whose nothern terminus was originally planned to be in South Hill, Virgina.

Map showing the proposed route of US 240 in Pennsylvania from 1927
The route of US 240 as it would have been designated in 1927.  (H. M. Gousha)
Southern Entrance: Maryland state line two miles south of Fairplay
Northern Terminus: US 11 at Market Street in Lemoyne
Length: 49.40 miles
Names: Emmitsburg Road, Steinwehr Avenue, Baltimore Street, Lincoln Square, Carlisle Street, East Lincoln Avenue, Harrisburg Street, Old Harrisburg Road, State Street, Capitol Hill Road, Range End Road, Gettysburg Street, South 2nd Street, West Harrisburg Street, East Harrisburg Street, North Chestnut Street, Harrisburg Pike, Gettysburg Pike, Gettysburg Road, State Road, and State Street
LR Designations: 298:  Maryland state line to Baltimore Street in Gettysburg
42:  Baltimore Street to Lincoln Avenue in Gettysburg
123:  Carlisle Street in Gettysburg to US 11 in Lemoyne
Counties: Adams, York, and Cumberland
Multiplexed Routes: None
Expressway: None
Former Designations:
None
Decommissioned: N/A: never posted
Replaced By: US 15
Links: US 15
End of Historic US Highway 240 - Dale Sanderson