Martin XO-4

{{short description|American observation aircraft proposal}}

{{Infobox aircraft

| name=XO-4

| image=

| image_caption=

| aircraft_type=Observation

| national_origin=United States

| manufacturer=Glenn L. Martin Company

| designer=

| first_flight=

| introduction=

| retired=

| status=Canceled

| primary_user=

| number_built=0

| developed_from=

| variants=

}}

The Martin XO-4 (Model 71) was a proposed observation aircraft designed by the Glenn L. Martin Company.{{Cite web |title=Martin Aircraft Model Designations, Built and Unbuilt |url=https://marylandaviationmuseum.org/pdf/Models.pdf |url-status=dead |archive-url=https://web.archive.org/web/20080517112813/https://marylandaviationmuseum.org/pdf/Models.pdf |archive-date=May 17, 2008 |website=Glenn L. Martin Maryland Aviation Museum}}{{Cite book|last=Duell|first=Sloan and Pearce|url=https://books.google.com/books?id=8uI1AQAAIAAJ&q=Martin+XO-4+aircraft&pg=RA3-PA7|title=The Official Pictorial History of the AAF|date=1947|publisher=United States Army Air Forces Historical Office|language=en}} It was designed to compete with the Curtiss XO-1, Douglas XO-2, and Dayton-Wright XO-3, and was to be powered by a Wright T-3 Tornado or a Liberty V-1650.{{Cite book|url=https://books.google.com/books?id=7Wk4AQAAMAAJ&q=Martin+XO-4+aircraft&pg=RA25-PT7|title=Air Service Information Circular|date=1923|publisher=Director of Air Service|language=en}}{{Cite web|date=2007-03-16|title=O - Observation Aircraft|url=http://www.personal.psu.edu/users/d/o/dob104/aviation/us/observe.html|access-date=2020-06-15|archive-url=https://web.archive.org/web/20070316084632/http://www.personal.psu.edu/users/d/o/dob104/aviation/us/observe.html|archive-date=2007-03-16}} Although it failed to receive a contract from the United States Navy, a single prototype (allocated the serial number 23-1255) was ordered by the United States Army Air Service, but this contract was canceled before the prototype could be built.{{Cite web|title=USAF Serial Number Search Results|url=http://cgibin.rcn.com/jeremy.k/cgi-bin/gzUsafSearch.pl?target=&content=XO-4|access-date=2020-06-15|website=cgibin.rcn.com}}